nft: fix interface comparisons in `-C` commands

nft: fix interface comparisons in `-C` commands

Commit 9ccae6397475 ("nft: Leave interface masks alone when parsing from
kernel") removed code which explicitly set interface masks to all ones.  The
result of this is that they are zero.  However, they are used to mask interfaces
in `is_same_interfaces`.  Consequently, the masked values are alway zero, the
comparisons are always true, and check commands which ought to fail succeed:

  # iptables -N test
  # iptables -A test -i lo \! -o lo -j REJECT
  # iptables -v -L test
  Chain test (0 references)
   pkts bytes target     prot opt in     out     source               destination
      0     0 REJECT     all  --  lo     !lo     anywhere             anywhere             reject-with icmp-port-unreachable
  # iptables -v -C test -i abcdefgh \! -o abcdefgh -j REJECT
  REJECT  all opt -- in lo out !lo  0.0.0.0/0  -> 0.0.0.0/0   reject-with icmp-port-unreachable

Remove the mask parameters from `is_same_interfaces`.  Add a test-case.

Fixes: 9ccae6397475 ("nft: Leave interface masks alone when parsing from kernel")
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Phil Sutter <phil@nwl.cc>