fs: switch bdev inode bdi's correctly

bdev inodes can remain dirty even after their last close. Hence the
BDI associated with the bdev->inode gets modified duringthe last
close to point to the default BDI. However, the bdev inode still
needs to be moved to the dirty lists of the new BDI, otherwise it
will corrupt the writeback list is was left on.

Add a new function bdev_inode_switch_bdi() to move all the bdi state
from the old bdi to the new one safely. This is only a temporary
measure until the bdev inode<->bdi lifecycle problems are sorted
out.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>