dm table: fix upgrade mode race

dm table: fix upgrade mode race

upstream commit: 570b9d968bf9b16974252ef7cbce73fa6dac34f3

upgrade_mode() sets bdev to NULL temporarily, and does not have any
locking to exclude anything from seeing that NULL.

In dm_table_any_congested() bdev_get_queue() can dereference that NULL and
cause a reported oops.

Fix this by not changing that field during the mode upgrade.

Cc: stable@kernel.org
Cc: Neil Brown <neilb@suse.de>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>