payload: do not kill dependency for proto_unknown

payload: do not kill dependency for proto_unknown

Unsupported meta match on layer 4 protocol sets on protocol context to
proto_unknown, handle anything coming after it as a raw expression in
payload_expr_expand().

Moreover, payload_dependency_kill() skips dependency removal if protocol
is unknown, so raw payload expression leaves meta layer 4 protocol
remains in place.

Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1641
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>