evalute: make vlan pcp updates work

evalute: make vlan pcp updates work

On kernel side, nft_payload_set_vlan() requires a 2 or 4 byte
write to the vlan header.

As-is, nft emits a 1 byte write:
  [ payload load 1b @ link header + 14 => reg 1 ]
  [ bitwise reg 1 = ( reg 1 & 0x0000001f ) ^ 0x00000020 ]

... which the kernel doesn't support.  Expand all vlan header updates to
a 2 or 4 byte write and update the existing vlan id test case.

Reported-by: Kevin Vigouroux <ke.vigouroux@laposte.net>
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Pablo Neira Ayuso <pablo@netfilter.org>