ipv6: don't stop backtracking in fib6_lookup_1 if subtree does not match

In case a subtree did not match we currently stop backtracking and return
NULL (root table from fib_lookup). This could yield in invalid routing
table lookups when using subtrees.

Instead continue to backtrack until a valid subtree or node is found
and return this match.

Also remove unneeded NULL check.
Reported-by: Teco Boot <teco@inf-net.nl>
Cc: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Cc: David Lamparter <equinox@diac24.net>
Cc: <boutier@pps.univ-paris-diderot.fr>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_fib.c

@@ -993,14 +993,22 @@ static struct fib6_node * fib6_lookup_1(struct fib6_node *root,
 
 			if (ipv6_prefix_equal(&key->addr, args->addr, key->plen)) {
 #ifdef CONFIG_IPV6_SUBTREES
-				if (fn->subtree)
-					fn = fib6_lookup_1(fn->subtree, args + 1);
+				if (fn->subtree) {
+					struct fib6_node *sfn;
+					sfn = fib6_lookup_1(fn->subtree,
+							    args + 1);
+					if (!sfn)
+						goto backtrack;
+					fn = sfn;
+				}
 #endif
-				if (!fn || fn->fn_flags & RTN_RTINFO)
+				if (fn->fn_flags & RTN_RTINFO)
 					return fn;
 			}
 		}
-
+#ifdef CONFIG_IPV6_SUBTREES
+backtrack:
+#endif
 		if (fn->fn_flags & RTN_ROOT)
 			break;