netlink_delinearize: also consider exthdr type when trimming binops

netlink_delinearize: also consider exthdr type when trimming binops

This allows trimming the binop for exthdrs, this will make nft render
   (tcp option mptcp unknown & 240) >> 4 . ip saddr @s1

as
    tcp option mptcp subtype . ip saddr @s1

Also extend the typeof set tests with a set concatenating a
sub-byte-sized exthdr expression with a payload one.

The additional call to expr_postprocess() is needed, without this,
typeof_sets_0.nft fails because
  frag frag-off @s4 accept

is shown as
 meta nfproto ipv6 frag frag-off @s4 accept

Previouly, EXPR_EXTHDR would cause payload_binop_postprocess()
to return false which will then make the caller invoke
expr_postprocess(), but after handling EXPR_EXTHDR this doesn't happen
anymore.

Signed-off-by: Florian Westphal <fw@strlen.de>