ceph: fix comments over ceph_add_cap

We actually need the ci->i_ceph_lock here. The necessity of the s_mutex
is less clear. Also add a lockdep assertion for the i_ceph_lock.

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