ipv6: fib: hide unused 'pn' variable

When CONFIG_IPV6_SUBTREES is disabled, the only user is hidden, causing
a 'make W=1' warning:

net/ipv6/ip6_fib.c: In function 'fib6_add':
net/ipv6/ip6_fib.c:1388:32: error: variable 'pn' set but not used [-Werror=unused-but-set-variable]

Add another #ifdef around the variable declaration, matching the other
uses in this file.

Fixes: 66729e1 ("[IPV6] ROUTE: Make sure we have fn->leaf when adding a node on subtree.")
Link: https://lore.kernel.org/netdev/20240322131746.904943-1-arnd@kernel.org/
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240408074219.3030256-1-arnd@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: arndb

net/ipv6/ip6_fib.c

@@ -1385,7 +1385,10 @@ int fib6_add(struct fib6_node *root, struct fib6_info *rt,
 	     struct nl_info *info, struct netlink_ext_ack *extack)
 {
 	struct fib6_table *table = rt->fib6_table;
-	struct fib6_node *fn, *pn = NULL;
+	struct fib6_node *fn;
+#ifdef CONFIG_IPV6_SUBTREES
+	struct fib6_node *pn = NULL;
+#endif
 	int err = -ENOMEM;
 	int allow_create = 1;
 	int replace_required = 0;
@@ -1409,9 +1412,9 @@ int fib6_add(struct fib6_node *root, struct fib6_info *rt,
 		goto out;
 	}
 
+#ifdef CONFIG_IPV6_SUBTREES
 	pn = fn;
 
-#ifdef CONFIG_IPV6_SUBTREES
 	if (rt->fib6_src.plen) {
 		struct fib6_node *sn;