ipv4: use IS_ENABLED instead of ifdef

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 b8127113d01e53adba15b41aefd37b90ed83d631.

Fixes: d15662682db2 ("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.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>