netlink: store set byteorder in NFTA_SET_USERDATA

netlink: store set byteorder in NFTA_SET_USERDATA

The integer datatype has neither specific byteorder nor length. This
results in the following broken output:

 # nft list ruleset
 table ip x {
        chain y {
                mark set cpu map { 0 : 0x00000001, 16777216 : 0x00000002}
        }
 }

Currently, with BYTEORDER_INVALID, nft defaults on network byteorder,
hence the output above.

This patch stores the key byteorder in the userdata using a TLV
structure in the NFTA_SET_USERDATA area, so nft can interpret key
accordingly when dumping the set back to userspace.

Thus, after this patch the listing is correct:

 # nft list ruleset
 table ip x {
        chain y {
                mark set cpu map { 0 : 0x00000001, 1 : 0x00000002}
        }
 }

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>