libceph: have cursor point to data

Rather than having a ceph message data item point to the cursor it's
associated with, have the cursor point to a data item.  This will
allow a message cursor to be used for more than one data item.
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

include/linux/ceph/messenger.h

@@ -104,13 +104,13 @@ struct ceph_msg_data {
 		};
 		struct ceph_pagelist	*pagelist;
 	};
-	struct ceph_msg_data_cursor	*cursor;
 };
 
 struct ceph_msg_data_cursor {
+	struct ceph_msg_data	*data;		/* data item this describes */
 	size_t			resid;		/* bytes not yet consumed */
-	bool		last_piece;	/* now at last piece of data item */
-	bool		need_crc;	/* new piece; crc update needed */
+	bool			last_piece;	/* current is last piece */
+	bool			need_crc;	/* crc update needed */
 	union {
 #ifdef CONFIG_BLOCK
 		struct {				/* bio */