ceph: drop private list from remove_session_caps_cb

This function does a lot of list-shuffling with cap flushes, all to
avoid possibly freeing a slab allocation under spinlock (which is
totally ok).  Simplify the code by just detaching and freeing the cap
flushes in place.

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