libnftnl vlan type filter

[Andreas Hoefler <andreas.hoefler@xxxxxxxxxxxxxxxxxxxxxx>]

Hi 
I am trying to use libnftnl to construct this:

table netdev filter {
        chain in {
                type filter hook ingress device pru20 priority 0; policy accept;
                vlan type 0x88ba
        }
}

I do :
    add_meta(r, NFT_META_IIFTYPE, NFT_REG_1);
    uint32_t iiftype = 1;
    add_cmp(r, NFT_REG_1, NFT_CMP_EQ, &iiftype, sizeof(iiftype));

    add_payload(r, NFT_PAYLOAD_LL_HEADER, NFT_REG_1, 12, sizeof(uint16_t));
    uint16_t vtype = htons(ETH_P_8021Q);
    add_cmp(r, NFT_REG_1, NFT_CMP_EQ, &vtype, sizeof(vtype));

    add_payload(r, NFT_PAYLOAD_LL_HEADER, NFT_REG_1, 16, sizeof(uint16_t));
    uint16_t et = htons(0x88ba);
    add_cmp(r, NFT_REG_1, NFT_CMP_EQ, &et, sizeof(et));

This produces the following rule
table netdev filter {
        chain in {
                type filter hook ingress device pru20 priority 0; policy drop;
                iiftype ether @ll,96,16 33024 @ll,128,16 35002
        }
}
When I manually add the constructed rule:
#nft add rule netdev filter in iiftype ether @ll,96,16 33024 @ll,128,16 35002

then nft list ruleset translates it correctly so I assume that this rule is built right:

table netdev filter {
        chain in {
                type filter hook ingress device pru20 priority 0; policy drop;
                iiftype ether @ll,96,16 33024 @ll,128,16 35002 <- constructed with code above
                vlan type 0x88ba <- manually added,  same rule as above but translated ok
        }
}

My questions:
- What are the correct enums to use for e.g iiftype =1;?
- Is there something like offsetof(struct ???, vlan) which I could use instead of hardcoded offset?
- Why does list ruleset show the coded rule differently from the manually added one?
- uint16_t vtype = htons(ETH_P_8021Q); seems weird to use htons here, is there another enum I should use?

Any help is highly appreaciated.
BR
Andy