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/include/linux/ceph/messenger.h b/include/linux/ceph/messenger.h
index de1d2e1..7c1420b 100644
--- a/include/linux/ceph/messenger.h
+++ b/include/linux/ceph/messenger.h
@@ -275,12 +275,12 @@
 
 extern void ceph_con_keepalive(struct ceph_connection *con);
 
-extern void ceph_msg_data_set_pages(struct ceph_msg *msg, struct page **pages,
+extern void ceph_msg_data_add_pages(struct ceph_msg *msg, struct page **pages,
 				size_t length, size_t alignment);
-extern void ceph_msg_data_set_pagelist(struct ceph_msg *msg,
+extern void ceph_msg_data_add_pagelist(struct ceph_msg *msg,
 				struct ceph_pagelist *pagelist);
 #ifdef CONFIG_BLOCK
-extern void ceph_msg_data_set_bio(struct ceph_msg *msg, struct bio *bio,
+extern void ceph_msg_data_add_bio(struct ceph_msg *msg, struct bio *bio,
 				size_t length);
 #endif /* CONFIG_BLOCK */