libceph: don't time out osd requests that haven't been received

Keep track of when an outgoing message is ACKed (i.e., the server fully
received it and, presumably, queued it for processing).  Time out OSD
requests only if it's been too long since they've been received.

This prevents timeouts and connection thrashing when the OSDs are simply
busy and are throttling the requests they read off the network.

Reviewed-by: Yehuda Sadeh <yehuda@hq.newdream.net>
Signed-off-by: Sage Weil <sage@newdream.net>

include/linux/ceph/messenger.h

@@ -94,6 +94,7 @@ struct ceph_msg {
 	bool more_to_follow;
 	bool needs_out_seq;
 	int front_max;
+	unsigned long ack_stamp;        /* tx: when we were acked */
 
 	struct ceph_msgpool *pool;
 };