xtables: Make invflags 16bit wide

xtables: Make invflags 16bit wide

This is needed to merge with xtables-arp which has more builtin
options and hence needs more bits in invflags.

The only adjustment needed is the set_option() call for option '-j'
which passed a pointer to cs->fw.ip.invflags. That field can't be
changed, it belongs to uAPI. Though using args->invflags instead works
fine, aside from that '-j' doesn't support inverting so this is merely a
sanity check and no real invflag value assignment will happen.

Signed-off-by: Phil Sutter <phil@nwl.cc>