bdev: Factor out bdev revalidation into a common helper

commit 731dc486 upstream.

Factor out code handling revalidation of bdev on disk change into a
common helper.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>