ceph: correct subscribe_ack msgpool payload size

Defined a struct for the SUBSCRIBE_ACK, and use that to size
the msgpool.
Signed-off-by: Sage Weil <sage@newdream.net>

fs/ceph/ceph_fs.h

@@ -162,6 +162,11 @@ struct ceph_mon_subscribe_item {
 	__u8 onetime;
 } __attribute__ ((packed));
 
+struct ceph_mon_subscribe_ack {
+	__le32 duration;         /* seconds */
+	struct ceph_fsid fsid;
+} __attribute__ ((packed));
+
 /*
  * mds states
  *   > 0 -> in

fs/ceph/mon_client.c

@@ -199,10 +199,12 @@ static void handle_subscribe_ack(struct ceph_mon_client *monc,
 				 struct ceph_msg *msg)
 {
 	unsigned seconds;
-	void *p = msg->front.iov_base;
-	void *end = p + msg->front.iov_len;
+	struct ceph_mon_subscribe_ack *h = msg->front.iov_base;
+
+	if (msg->front.iov_len < sizeof(*h))
+		goto bad;
+	seconds = le32_to_cpu(h->duration);
 
-	ceph_decode_32_safe(&p, end, seconds, bad);
 	mutex_lock(&monc->mutex);
 	if (monc->hunting) {
 		pr_info("mon%d %s session established\n",
@@ -541,7 +543,8 @@ int ceph_monc_init(struct ceph_mon_client *monc, struct ceph_client *cl)
 	err = ceph_msgpool_init(&monc->msgpool_mount_ack, 4096, 1, false);
 	if (err < 0)
 		goto out;
-	err = ceph_msgpool_init(&monc->msgpool_subscribe_ack, 8, 1, false);
+	err = ceph_msgpool_init(&monc->msgpool_subscribe_ack,
+			       sizeof(struct ceph_mon_subscribe_ack), 1, false);
 	if (err < 0)
 		goto out;
 	err = ceph_msgpool_init(&monc->msgpool_statfs_reply,