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>
diff --git a/fs/ceph/ceph_fs.h b/fs/ceph/ceph_fs.h
index 56af192..9b16e2e 100644
--- a/fs/ceph/ceph_fs.h
+++ b/fs/ceph/ceph_fs.h
@@ -162,6 +162,11 @@
 	__u8 onetime;
 } __attribute__ ((packed));
 
+struct ceph_mon_subscribe_ack {
+	__le32 duration;         /* seconds */
+	struct ceph_fsid fsid;
+} __attribute__ ((packed));
+
 /*
  * mds states
  *   > 0 -> in