ceph: make __take_cap_refs non-static

Rename it to ceph_take_cap_refs and make it available to other files.
Also replace a comment with a lockdep assertion.

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