libceph: verify state after retaking con lock after dispatch

We drop the con mutex when delivering a message.  When we retake the
lock, we need to verify we are still in the OPEN state before
preparing to read the next tag, or else we risk stepping on a
connection that has been closed.
Signed-off-by: NSage Weil <sage@inktank.com>
Reviewed-by: NAlex Elder <elder@inktank.com>

net/ceph/messenger.c

@@ -2003,7 +2003,6 @@ static void process_message(struct ceph_connection *con)
 	con->ops->dispatch(con, msg);
 
 	mutex_lock(&con->mutex);
-	prepare_read_tag(con);
 }
 
 
@@ -2213,6 +2212,8 @@ static int try_read(struct ceph_connection *con)
 		if (con->in_tag == CEPH_MSGR_TAG_READY)
 			goto more;
 		process_message(con);
+		if (con->state == CON_STATE_OPEN)
+			prepare_read_tag(con);
 		goto more;
 	}
 	if (con->in_tag == CEPH_MSGR_TAG_ACK) {