ceph: take a cred reference instead of tracking individual uid/gid

Replace req->r_uid/r_gid with an r_cred pointer and take a reference to
that at the point where we previously would sample the two.  Use that to
populate the uid and gid in the header and release the reference when
the request is freed.

This should enable us to later add support for sending supplementary
group lists in MDS requests.

[ idryomov: break unnecessarily long lines ]

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>