netfilter: xt_socket: Make tproto signed in socket_mt6_v1().

Otherwise error indications from ipv6_find_hdr() won't be noticed.

This required making the protocol argument to extract_icmp6_fields()
signed too.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: David S. Miller <davem@davemloft.net>