libceph: behave in mon_fault() if cur_mon < 0

This can happen if __close_session() in ceph_monc_stop() races with
a connection reset.  We need to ignore such faults, otherwise it's
likely we would take !hunting, call __schedule_delayed() and end up
with delayed_work() executing on invalid memory, among other things.

The (two!) con->private tests are useless, as nothing ever clears
con->private.  Nuke them.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

net/ceph/mon_client.c

@@ -1155,14 +1155,9 @@ static void mon_fault(struct ceph_connection *con)
 {
 	struct ceph_mon_client *monc = con->private;
 
-	if (!monc)
-		return;
-
-	dout("mon_fault\n");
 	mutex_lock(&monc->mutex);
-	if (!con->private)
-		goto out;
-
+	dout("%s mon%d\n", __func__, monc->cur_mon);
+	if (monc->cur_mon >= 0) {
 		if (!monc->hunting) {
 			dout("%s hunting for new mon\n", __func__);
 			reopen_session(monc);
@@ -1170,7 +1165,7 @@ static void mon_fault(struct ceph_connection *con)
 		} else {
 			dout("%s already hunting\n", __func__);
 		}
-out:
+	}
 	mutex_unlock(&monc->mutex);
 }