ceph: use kill_anon_super helper

ceph open-codes this around some other activity and the rationale
for it isn't clear. There is no need to delay free_anon_bdev until
the end of kill_sb.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>