extensions: libipt_icmp: Fix translation of type 'any'

extensions: libipt_icmp: Fix translation of type 'any'

By itself, '-m icmp --icmp-type any' is a noop, it matches any icmp
types. Yet nft_ipv4_xlate() does not emit an 'ip protocol' match if
there's an extension with same name present in the rule. Luckily, legacy
iptables demands icmp match to be prepended by '-p icmp', so we can
assume this is present and just emit the 'ip protocol' match from icmp
xlate callback.

Fixes: aa158ca0fda65 ("extensions: libipt_icmp: Add translation to nft")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Reviewed-by: Florian Westphal <fw@strlen.de>