[IPV6]: Clean up BACKTRACK().

The fn check is unnecessary as fn can never be NULL in BACKTRACK().
Signed-off-by: NVille Nuorvala <vnuorval@tcs.hut.fi>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -484,7 +484,7 @@ int rt6_route_rcv(struct net_device *dev, u8 *opt, int len,
 do { \
 	if (rt == &ip6_null_entry) { \
 		struct fib6_node *pn; \
-		while (fn) { \
+		while (1) { \
 			if (fn->fn_flags & RTN_TL_ROOT) \
 				goto out; \
 			pn = fn->parent; \