ceph: eliminate req->r_wait_for_completion from ceph_mds_request

...and instead just pass the wait function on the stack.

Make ceph_mdsc_wait_request non-static, and add an argument for wait for
completion. Then have ceph_lock_message call ceph_mdsc_submit_request,
and ceph_mdsc_wait_request and pass in the pointer to
ceph_lock_wait_for_completion.

While we're in there, rearrange some fields in ceph_mds_request, so we
save a total of 24 bytes per.

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