ceph: have ceph_mdsc_do_request call ceph_mdsc_submit_request

Nothing calls ceph_mdsc_submit_request today, but in later patches we'll
need to be able to call this separately.

Have the helper return an int so we can check the r_err under the mutex,
and have the caller just check the error code from the submit. Also move
the acquisition of CEPH_CAP_PIN references into the same function.

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