* util/linuxfw: fix IPv6 NAT availability check for nftables
When running firewall in nftables mode,
there is no need for a separate NAT availability check
(unlike with iptables, there are no hosts that support nftables, but not IPv6 NAT - see tailscale/tailscale#11353).
This change fixes a firewall NAT availability check that was using the no-longer set ipv6NATAvailable field
by removing the field and using a method that, for nftables, just checks that IPv6 is available.
Updates tailscale/tailscale#12008
Signed-off-by: Irbe Krumina <irbe@tailscale.com>
7ef2f72135