ceph: after an MDS request, do callback and completions

No MDS requests use r_callback today, but that will change in the
future. The OSD client always does r_callback and then completes
r_completion. Let's have the MDS client do the same.

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