libceph: define and use an explicit CONNECTED state

There is no state explicitly defined when a ceph connection is fully
operational.  So define one.

It's set when the connection sequence completes successfully, and is
cleared when the connection gets closed.

Be a little more careful when examining the old state when a socket
disconnect event is reported.

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

include/linux/ceph/messenger.h

@@ -120,6 +120,7 @@ struct ceph_msg_pos {
  */
 #define CONNECTING	1
 #define NEGOTIATING	2
+#define CONNECTED	5
 #define STANDBY		8  /* no outgoing messages, socket closed.  we keep
 			    * the ceph_connection around to maintain shared
 			    * state with the peer. */