arptables: Fix conditional opcode/proto-type printing

arptables: Fix conditional opcode/proto-type printing

The checks were wrong: nft_arp_init_cs() initializes masks to 65535, not
0. This went on unnoticed because nft_arp_add() does it right and
init_cs callback was not used in e.g. nft_arp_print_rule(). The last
patch adding init_cs() calls in potentially required spots exposed this
though.

Fixes: 84909d171585d ("xtables: bootstrap ARP compatibility layer for nftables")
Signed-off-by: Phil Sutter <phil@nwl.cc>