ipv6: check fn before doing FIB6_SUBTREE(fn)

In fib6_locate(), we need to first make sure fn is not NULL before doing
FIB6_SUBTREE(fn) to avoid crash.

This fixes the following static checker warning:
net/ipv6/ip6_fib.c:1462 fib6_locate()
         warn: variable dereferenced before check 'fn' (see line 1459)

net/ipv6/ip6_fib.c
  1458          if (src_len) {
  1459                  struct fib6_node *subtree = FIB6_SUBTREE(fn);
                                                    ^^^^^^^^^^^^^^^^
We shifted this dereference

  1460
  1461                  WARN_ON(saddr == NULL);
  1462                  if (fn && subtree)
                            ^^
before the check for NULL.

  1463                          fn = fib6_locate_1(subtree, saddr, src_len,
  1464                                             offsetof(struct rt6_info, rt6i_src)

Fixes: 66f5d6ce ("ipv6: replace rwlock with rcu and spinlock in fib6_table")
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NWei Wang <weiwan@google.com>
Acked-by: NEric Dumazet <edumazet@google.com>
Acked-by: NMartin KaFai Lau <kafai@fb.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ip6_fib.c

@@ -1456,13 +1456,16 @@ struct fib6_node *fib6_locate(struct fib6_node *root,
 
 #ifdef CONFIG_IPV6_SUBTREES
 	if (src_len) {
-		struct fib6_node *subtree = FIB6_SUBTREE(fn);
-
 		WARN_ON(saddr == NULL);
-		if (fn && subtree)
-			fn = fib6_locate_1(subtree, saddr, src_len,
+		if (fn) {
+			struct fib6_node *subtree = FIB6_SUBTREE(fn);
+
+			if (subtree) {
+				fn = fib6_locate_1(subtree, saddr, src_len,
 					   offsetof(struct rt6_info, rt6i_src),
 					   exact_match);
+			}
+		}
 	}
 #endif