src: ipv6: switch implicit dependencies to meta l4proto

src: ipv6: switch implicit dependencies to meta l4proto

when using rule like

ip6 filter input tcp dport 22
nft generates:
  [ payload load 1b @ network header + 6 => reg 1 ]
  [ cmp eq reg 1 0x00000006 ]
  [ payload load 2b @ transport header + 2 => reg 1 ]
  [ cmp eq reg 1 0x00001600 ]

which is: ip6 filter input ip6 nexthdr tcp dport 22
IOW, such a rule won't match if e.g. a fragment header is in place.

This changes ip6_proto to use 'meta l4proto' which is the protocol header
found by exthdr walk.

A side effect is that for bridge we get a shorter dependency chain as it
no longer needs to prepend 'ether proto ipv6' for old 'ip6 nexthdr' dep.

Only problem:

ip6 nexthdr tcp tcp dport 22
will now inject a (useless) meta l4 dependency as ip6 nexthdr is no
longer flagged as EXPR_F_PROTOCOL, to avoid this add a small helper
that skips the unneded meta dependency in that case.

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