nft: Fix add_bitwise_u16() on Big Endian

nft: Fix add_bitwise_u16() on Big Endian

Type used for 'mask' and 'xor' parameters was wrong, 'int' is four bytes
on 32 or 64 bit architectures. After casting a uint16_t to int, on Big
Endian the first two bytes of data are (the leading) zero which libnftnl
then copies instead of the actual value.

This problem was noticed when using '--fragment' option:

| # iptables-nft -A FORWARD --fragment -j ACCEPT
| # nft list ruleset | grep frag-off
| ip frag-off & 0 != 0 counter packets 0 bytes 0 accept

With this fix in place, the resulting nft rule is correct:

| ip frag-off & 8191 != 0 counter packets 0 bytes 0 accept

Fixes: 2f1fbab671576 ("iptables: nft: add -f support")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>