libceph: have messages point to their connection

When a ceph message is queued for sending it is placed on a list of
pending messages (ceph_connection->out_queue).  When they are
actually sent over the wire, they are moved from that list to
another (ceph_connection->out_sent).  When acknowledgement for the
message is received, it is removed from the sent messages list.

During that entire time the message is "in the possession" of a
single ceph connection.  Keep track of that connection in the
message.  This will be used in the next patch (and is a helpful
bit of information for debugging anyway).

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>

include/linux/ceph/messenger.h

@@ -77,7 +77,10 @@ struct ceph_msg {
 	unsigned nr_pages;              /* size of page array */
 	unsigned page_alignment;        /* io offset in first page */
 	struct ceph_pagelist *pagelist; /* instead of pages */
+
+	struct ceph_connection *con;
 	struct list_head list_head;
+
 	struct kref kref;
 	struct bio  *bio;		/* instead of pages/pagelist */
 	struct bio  *bio_iter;		/* bio iterator */