libceph: set message data when building osd request

All calls of ceph_osdc_start_request() are preceded (in the case of
rbd, almost) immediately by a call to ceph_osdc_build_request().

Move the build calls at the top of ceph_osdc_start_request() out of
there and into the ceph_osdc_build_request().  Nothing prevents
moving these calls to the top of ceph_osdc_build_request(), either
(and we're going to want them there in the next patch) so put them
at the top.

This and the next patch are related to:
    http://tracker.ceph.com/issues/4657Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

net/ceph/osd_client.c

@@ -1884,6 +1884,11 @@ void ceph_osdc_build_request(struct ceph_osd_request *req, u64 off,
 	u64 data_len;
 	unsigned int i;
 
+	/* Set up response incoming data and request outgoing data fields */
+
+	ceph_osdc_msg_data_set(req->r_reply, &req->r_data_in);
+	ceph_osdc_msg_data_set(req->r_request, &req->r_data_out);
+
 	req->r_snapid = snap_id;
 	req->r_snapc = ceph_get_snap_context(snapc);
 
@@ -1976,11 +1981,6 @@ int ceph_osdc_start_request(struct ceph_osd_client *osdc,
 {
 	int rc = 0;
 
-	/* Set up response incoming data and request outgoing data fields */
-
-	ceph_osdc_msg_data_set(req->r_reply, &req->r_data_in);
-	ceph_osdc_msg_data_set(req->r_request, &req->r_data_out);
-
 	down_read(&osdc->map_sem);
 	mutex_lock(&osdc->request_mutex);
 	__register_request(osdc, req);