ipv4: use IS_ENABLED instead of ifdef

Checking for ifdef CONFIG_x fails if CONFIG_x=m.

Use IS_ENABLED instead, which is true for both built-ins and modules.

Otherwise, a
> ip -4 route add 1.2.3.4/32 via inet6 fe80::2 dev eth1
fails with the message "Error: IPv6 support not enabled in kernel." if
CONFIG_IPV6 is `m`.

In the spirit of b8127113.

Fixes: d1566268 ("ipv4: Allow ipv6 gateway with ipv4 routes")
Cc: Kim Phillips <kim.phillips@arm.com>
Signed-off-by: Florian Klink <flokli@flokli.de>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20201115224509.2020651-1-flokli@flokli.deSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/fib_frontend.c

@@ -696,7 +696,7 @@ int fib_gw_from_via(struct fib_config *cfg, struct nlattr *nla,
 		cfg->fc_gw4 = *((__be32 *)via->rtvia_addr);
 		break;
 	case AF_INET6:
-#ifdef CONFIG_IPV6
+#if IS_ENABLED(CONFIG_IPV6)
 		if (alen != sizeof(struct in6_addr)) {
 			NL_SET_ERR_MSG(extack, "Invalid IPv6 address in RTA_VIA");
 			return -EINVAL;