ceph: reuse mon subscribe message instead of allocated anew

Use the same message, allocated during startup.  No need to reallocate a
new one each time around (and potentially ENOMEM).

Signed-off-by: Sage Weil <sage@newdream.net>
diff --git a/fs/ceph/mon_client.h b/fs/ceph/mon_client.h
index 7688778..174d794 100644
--- a/fs/ceph/mon_client.h
+++ b/fs/ceph/mon_client.h
@@ -63,7 +63,7 @@
 	struct delayed_work delayed_work;
 
 	struct ceph_auth_client *auth;
-	struct ceph_msg *m_auth, *m_auth_reply, *m_subscribe_ack;
+	struct ceph_msg *m_auth, *m_auth_reply, *m_subscribe, *m_subscribe_ack;
 	int pending_auth;
 
 	bool hunting;