ieee802154: 6lowpan_rtnl: fix correct errno value

This patch correct the return value of lowpan_alloc_frag if an error
occur. Errno numbers should always be negative.
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/ieee802154/6lowpan_rtnl.c

@@ -246,7 +246,7 @@ lowpan_alloc_frag(struct sk_buff *skb, int size,
 			return ERR_PTR(-rc);
 		}
 	} else {
-		frag = ERR_PTR(ENOMEM);
+		frag = ERR_PTR(-ENOMEM);
 	}
 
 	return frag;