libceph: add, don't set data for a message

Change the names of the functions that put data on a pagelist to
reflect that we're adding to whatever's already there rather than
just setting it to the one thing.  Currently only one data item is
ever added to a message, but that's about to change.

This resolves:
    http://tracker.ceph.com/issues/2770

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
index 13ae44e..4f22671 100644
--- a/fs/ceph/mds_client.c
+++ b/fs/ceph/mds_client.c
@@ -1724,7 +1724,7 @@
 	if (req->r_data_len) {
 		/* outbound data set only by ceph_sync_setxattr() */
 		BUG_ON(!req->r_pages);
-		ceph_msg_data_set_pages(msg, req->r_pages, req->r_data_len, 0);
+		ceph_msg_data_add_pages(msg, req->r_pages, req->r_data_len, 0);
 	}
 
 	msg->hdr.data_len = cpu_to_le32(req->r_data_len);
@@ -2608,7 +2608,7 @@
 	if (pagelist->length) {
 		/* set up outbound data if we have any */
 		reply->hdr.data_len = cpu_to_le32(pagelist->length);
-		ceph_msg_data_set_pagelist(reply, pagelist);
+		ceph_msg_data_add_pagelist(reply, pagelist);
 	}
 	ceph_con_send(&session->s_con, reply);