xen/9pfs: initialize len to 0 to detect xenbus_read errors

In order to use "len" to check for xenbus_read errors properly, we need
to initialize len to 0 before passing it to xenbus_read.

CC: dan.carpenter@oracle.com
CC: jgross@suse.com
CC: boris.ostrovsky@oracle.com
CC: Eric Van Hensbergen <ericvh@gmail.com>
CC: Ron Minnich <rminnich@sandia.gov>
CC: Latchesar Ionkov <lucho@ionkov.net>
CC: v9fs-developer@lists.sourceforge.net
Signed-off-by: Stefano Stabellini <stefano@aporeto.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

net/9p/trans_xen.c

@@ -386,7 +386,7 @@ static int xen_9pfs_front_probe(struct xenbus_device *dev,
 	struct xenbus_transaction xbt;
 	struct xen_9pfs_front_priv *priv = NULL;
 	char *versions;
-	unsigned int max_rings, max_ring_order, len;
+	unsigned int max_rings, max_ring_order, len = 0;
 
 	versions = xenbus_read(XBT_NIL, dev->otherend, "versions", &len);
 	if (!len)