netfilter: xt_socket: fix a stack corruption bug

As soon as extract_icmp6_fields() returns, its local storage (automatic
variables) is deallocated and can be overwritten.

Lets add an additional parameter to make sure storage is valid long
enough.

While we are at it, adds some const qualifiers.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Fixes: b64c9256 ("tproxy: added IPv6 support to the socket match")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>