fs: Fix freeze_bdev()/thaw_bdev() accounting of bd_fsfreeze_sb

freeze/thaw_bdev() currently use bdev->bd_fsfreeze_count to infer
whether or not bdev->bd_fsfreeze_sb is valid (it's valid iff
bd_fsfreeze_count is non-zero). thaw_bdev() doesn't nullify
bd_fsfreeze_sb.

But this means a freeze_bdev() call followed by a thaw_bdev() call can
leave bd_fsfreeze_sb with a non-null value, while bd_fsfreeze_count is
zero. If freeze_bdev() is called again, and this time
get_active_super() returns NULL (e.g. because the FS is unmounted),
we'll end up with bd_fsfreeze_count > 0, but bd_fsfreeze_sb is
*untouched* - it stays the same (now garbage) value. A subsequent
thaw_bdev() will decide that the bd_fsfreeze_sb value is legitimate
(since bd_fsfreeze_count > 0), and attempt to use it.

Fix this by always setting bd_fsfreeze_sb to NULL when
bd_fsfreeze_count is successfully decremented to 0 in thaw_sb().
Alternatively, we could set bd_fsfreeze_sb to whatever
get_active_super() returns in freeze_bdev() whenever bd_fsfreeze_count
is successfully incremented to 1 from 0 (which can be achieved cleanly
by moving the line currently setting bd_fsfreeze_sb to immediately
after the "sync:" label, but it might be a little too subtle/easily
overlooked in future).

This fixes the currently panicking xfstests generic/085.

Fixes: 040f04bd

 ("fs: simplify freeze_bdev/thaw_bdev")
Signed-off-by: Satya Tangirala <satyat@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>