libceph: isolate message page field manipulation

Define a function ceph_msg_data_set_pages(), which more clearly
abstracts the assignment page-related fields for data in a ceph
message structure.  Use this new function in the osd client and mds
client.

Ideally, these fields would never be set more than once (with
BUG_ON() calls to guarantee that).  At the moment though the osd
client sets these every time it receives a message, and in the event
of a communication problem this can happen more than once.  (This
will be resolved shortly, but setting up these helpers first makes
it all a bit easier to work with.)

Rearrange the field order in a ceph_msg structure to group those
that are used to define the possible data payloads.

This partially resolves:
    http://tracker.ceph.com/issues/4263

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
diff --git a/net/ceph/messenger.c b/net/ceph/messenger.c
index ce1669f..cec39cb 100644
--- a/net/ceph/messenger.c
+++ b/net/ceph/messenger.c
@@ -2689,6 +2689,17 @@
 }
 EXPORT_SYMBOL(ceph_con_keepalive);
 
+void ceph_msg_data_set_pages(struct ceph_msg *msg, struct page **pages,
+		unsigned int page_count, size_t alignment)
+{
+	/* BUG_ON(msg->pages); */
+	/* BUG_ON(msg->page_count); */
+
+	msg->pages = pages;
+	msg->page_count = page_count;
+	msg->page_alignment = alignment & ~PAGE_MASK;
+}
+EXPORT_SYMBOL(ceph_msg_data_set_pages);
 
 /*
  * construct a new message with given type, size
diff --git a/net/ceph/osd_client.c b/net/ceph/osd_client.c
index 202af14..a09d571 100644
--- a/net/ceph/osd_client.c
+++ b/net/ceph/osd_client.c
@@ -1760,11 +1760,10 @@
 	if (osd_data->type == CEPH_OSD_DATA_TYPE_PAGES) {
 		unsigned int page_count;
 
-		req->r_request->pages = osd_data->pages;
 		page_count = calc_pages_for((u64)osd_data->alignment,
 						(u64)osd_data->length);
-		req->r_request->page_count = page_count;
-		req->r_request->page_alignment = osd_data->alignment;
+		ceph_msg_data_set_pages(req->r_request, osd_data->pages,
+			page_count, osd_data->alignment);
 #ifdef CONFIG_BLOCK
 	} else if (osd_data->type == CEPH_OSD_DATA_TYPE_BIO) {
 		req->r_request->bio = osd_data->bio;
@@ -2135,9 +2134,8 @@
 			}
 			page_count = calc_pages_for((u64)osd_data->alignment,
 							(u64)osd_data->length);
-			m->pages = osd_data->pages;
-			m->page_count = page_count;
-			m->page_alignment = osd_data->alignment;
+			ceph_msg_data_set_pages(m, osd_data->pages,
+				osd_data->num_pages, osd_data->alignment);
 #ifdef CONFIG_BLOCK
 		} else if (osd_data->type == CEPH_OSD_DATA_TYPE_BIO) {
 			m->bio = osd_data->bio;