ceph: clean up spinlocking and list handling around cleanup_cap_releases()

Functions that release a lock taken in a parent frame are notoriously
hard to follow. Split cleanup_cap_releases into two functions, one to
detach the cap releases from the session (which should be called with
the spinlock held), and another to dispose of those caps.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>