netfilter: xt_socket: fix a typo in socket_mt_destroy()

Calling nf_defrag_ipv4_disable() instead of nf_defrag_ipv6_disable()
was probably not the intent.

I found this by code inspection, while chasing a possible issue in TPROXY.

Fixes: de8c1211

 ("netfilter: disable defrag once its no longer needed")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>