libceph: implement bio message data item cursor

Implement and use cursor routines for bio message data items for
outbound message data.

(See the previous commit for reasoning in support of the changes
in out_msg_pos_next().)

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

include/linux/ceph/messenger.h

@@ -98,6 +98,13 @@ static __inline__ bool ceph_msg_data_type_valid(enum ceph_msg_data_type type)
 struct ceph_msg_data_cursor {
 	bool		last_piece;	/* now at last piece of data item */
 	union {
+#ifdef CONFIG_BLOCK
+		struct {				/* bio */
+			struct bio	*bio;		/* bio from list */
+			unsigned int	vector_index;	/* vector from bio */
+			unsigned int	vector_offset;	/* bytes from vector */
+		};
+#endif /* CONFIG_BLOCK */
 		struct {				/* pagelist */
 			struct page	*page;		/* page from list */
 			size_t		offset;		/* bytes from list */