netifd: interface-ip: don't set fib6 policies if ipv6 disabled

netifd: interface-ip: don't set fib6 policies if ipv6 disabled

If IPv6 is disabled on a device, netifd still creates rules for it:

0:      from all lookup local
32766:  from all lookup main
4200000001:     from all iif lo lookup unspec 12
4200000002:     from all iif eth0 lookup unspec 12
4200000003:     from all iif eth1 lookup unspec 12

When logread is asked to log to a remote system, it invokes usock such
that getaddrinfo is called with AI_ADDRCONFIG in the flags; if ipv6 is
disabled on lo, musl attempts to connect to ::1 but gets EACCES from
the kernel, because of the reject policy added; this causes logread to
fail to connect:

socket(AF_INET6, SOCK_DGRAM|SOCK_CLOEXEC, IPPROTO_UDP) = 8
connect(8, {sa_family=AF_INET6, sin6_port=htons(65535), sin6_flowinfo=htonl(0), inet_pton(AF_INET6, "::1", &sin6_addr), sin6_scope_id=0}, 28) = -1 EACCES (Permission denied)

See <https://www.openwall.com/lists/musl/2021/04/30/2> for a
discussion of musl's handling of this.

This change only sets up the v6 rules if ipv6 is enabled on the
device.

Signed-off-by: Julian Squires <julian@cipht.net>