{vxlan, inet6} Mark vxlan_dev flags with VXLAN_F_IPV6 properly

{vxlan, inet6} Mark vxlan_dev flags with VXLAN_F_IPV6 properly

Even if user doesn't supply the physical netdev to attach vxlan dev
to, and at the same time user want to vxlan sit top of IPv6, mark
vxlan_dev flags with VXLAN_F_IPV6 to create IPv6 based socket.
Otherwise kernel crashes safely every time spitting below messages,

Steps to reproduce:
ip link add vxlan0 type vxlan id 42 group ff0e::110
ip link set vxlan0 up

[   62.656266] BUG: unable to handle kernel NULL pointer dereference[   62.656320] ip (3008) used greatest stack depth: 3912 bytes left
 at 0000000000000046
[   62.656423] IP: [<ffffffff816d822d>] ip6_route_output+0xbd/0xe0
[   62.656525] PGD 2c966067 PUD 2c9a2067 PMD 0
[   62.656674] Oops: 0000 [#1] SMP
[   62.656781] Modules linked in: vxlan netconsole deflate zlib_deflate af_key
[   62.657083] CPU: 1 PID: 2128 Comm: whoopsie Not tainted 3.12.0+ #182
[   62.657083] Hardware name: innotek GmbH VirtualBox, BIOS VirtualBox 12/01/2006
[   62.657083] task: ffff88002e2335d0 ti: ffff88002c94c000 task.ti: ffff88002c94c000
[   62.657083] RIP: 0010:[<ffffffff816d822d>]  [<ffffffff816d822d>] ip6_route_output+0xbd/0xe0
[   62.657083] RSP: 0000:ffff88002fd038f8  EFLAGS: 00210296
[   62.657083] RAX: 0000000000000000 RBX: ffff88002fd039e0 RCX: 0000000000000000
[   62.657083] RDX: ffff88002fd0eb68 RSI: ffff88002fd0d278 RDI: ffff88002fd0d278
[   62.657083] RBP: ffff88002fd03918 R08: 0000000002000000 R09: 0000000000000000
[   62.657083] R10: 00000000000001ff R11: 0000000000000000 R12: 0000000000000001
[   62.657083] R13: ffff88002d96b480 R14: ffffffff81c8e2c0 R15: 0000000000000001
[   62.657083] FS:  0000000000000000(0000) GS:ffff88002fd00000(0063) knlGS:00000000f693b740
[   62.657083] CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
[   62.657083] CR2: 0000000000000046 CR3: 000000002c9d2000 CR4: 00000000000006e0
[   62.657083] Stack:
[   62.657083]  ffff88002fd03a40 ffffffff81c8e2c0 ffff88002fd039e0 ffff88002d96b480
[   62.657083]  ffff88002fd03958 ffffffff816cac8b ffff880019277cc0 ffff8800192b5d00
[   62.657083]  ffff88002d5bc000 ffff880019277cc0 0000000000001821 0000000000000001
[   62.657083] Call Trace:
[   62.657083]  <IRQ>
[   62.657083]  [<ffffffff816cac8b>] ip6_dst_lookup_tail+0xdb/0xf0
[   62.657083]  [<ffffffff816caea0>] ip6_dst_lookup+0x10/0x20
[   62.657083]  [<ffffffffa0020c13>] vxlan_xmit_one+0x193/0x9c0 [vxlan]
[   62.657083]  [<ffffffff8137b3b7>] ? account+0xc7/0x1f0
[   62.657083]  [<ffffffffa0021513>] vxlan_xmit+0xd3/0x400 [vxlan]
[   62.657083]  [<ffffffff8161390d>] dev_hard_start_xmit+0x49d/0x5e0
[   62.657083]  [<ffffffff81613d29>] dev_queue_xmit+0x2d9/0x480
[   62.657083]  [<ffffffff817cb854>] ? _raw_write_unlock_bh+0x14/0x20
[   62.657083]  [<ffffffff81630565>] ? eth_header+0x35/0xe0
[   62.657083]  [<ffffffff8161bc5e>] neigh_resolve_output+0x11e/0x1e0
[   62.657083]  [<ffffffff816ce0e0>] ? ip6_fragment+0xad0/0xad0
[   62.657083]  [<ffffffff816cb465>] ip6_finish_output2+0x2f5/0x470
[   62.657083]  [<ffffffff816ce166>] ip6_finish_output+0x86/0xc0
[   62.657083]  [<ffffffff816ce218>] ip6_output+0x78/0xb0
[   62.657083]  [<ffffffff816eadd6>] mld_sendpack+0x256/0x2a0
[   62.657083]  [<ffffffff816ebd8c>] mld_ifc_timer_expire+0x17c/0x290
[   62.657083]  [<ffffffff816ebc10>] ? igmp6_timer_handler+0x80/0x80
[   62.657083]  [<ffffffff816ebc10>] ? igmp6_timer_handler+0x80/0x80
[   62.657083]  [<ffffffff81051065>] call_timer_fn+0x45/0x150
[   62.657083]  [<ffffffff816ebc10>] ? igmp6_timer_handler+0x80/0x80
[   62.657083]  [<ffffffff81052353>] run_timer_softirq+0x1f3/0x2a0
[   62.657083]  [<ffffffff8102dfd8>] ? lapic_next_event+0x18/0x20
[   62.657083]  [<ffffffff8109e36f>] ? clockevents_program_event+0x6f/0x110
[   62.657083]  [<ffffffff8104a2f6>] __do_softirq+0xd6/0x2b0
[   62.657083]  [<ffffffff8104a75e>] irq_exit+0x7e/0xa0
[   62.657083]  [<ffffffff8102ea15>] smp_apic_timer_interrupt+0x45/0x60
[   62.657083]  [<ffffffff817d3eca>] apic_timer_interrupt+0x6a/0x70
[   62.657083]  <EOI>
[   62.657083]  [<ffffffff817d4a35>] ? sysenter_dispatch+0x7/0x1a
[   62.657083] Code: 4d 8b 85 a8 02 00 00 4c 89 e9 ba 03 04 00 00 48 c7 c6 c0 be 8d 81 48 c7 c7 48 35 a3 81 31 c0 e8 db 68 0e 00 49 8b 85 a8 02 00 00 <0f> b6 40 46 c0 e8 05 0f b6 c0 c1 e0 03 41 09 c4 e9 77 ff ff ff
[   62.657083] RIP  [<ffffffff816d822d>] ip6_route_output+0xbd/0xe0
[   62.657083]  RSP <ffff88002fd038f8>
[   62.657083] CR2: 0000000000000046
[   62.657083] ---[ end trace ba8a9583d7cd1934 ]---
[   62.657083] Kernel panic - not syncing: Fatal exception in interrupt

Signed-off-by: Fan Du <fan.du@windriver.com>
Reported-by: Ryan Whelan <rcwhelan@gmail.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>