Btrfs: fix compile warnings on on avr32 platform

Btrfs: fix compile warnings on on avr32 platform

fs/btrfs/scrub.c: In function 'get_raid56_logic_offset':
fs/btrfs/scrub.c:2269: warning: comparison of distinct pointer types lacks a cast
fs/btrfs/scrub.c:2269: warning: right shift count >= width of type
fs/btrfs/scrub.c:2269: warning: passing argument 1 of '__div64_32' from incompatible pointer type

Since @rot is an int type, we should not use do_div(), fix it.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>