netfilter: fix module dependency issues with IPv6 defragmentation, ip6tables and xt_TPROXY

One of the previous tproxy related patches split IPv6 defragmentation and
connection tracking, but did not correctly add Kconfig stanzas to handle the
new dependencies correctly. This patch fixes that by making the config options
mirror the setup we have for IPv4: a distinct config option for defragmentation
that is automatically selected by both connection tracking and
xt_TPROXY/xt_socket.

The patch also changes the #ifdefs enclosing IPv6 specific code in xt_socket
and xt_TPROXY: we only compile these in case we have ip6tables support enabled.

Signed-off-by: KOVACS Krisztian <hidden@balabit.hu>
Signed-off-by: David S. Miller <davem@davemloft.net>