netfilter: nf_defrag_ipv6: solve section mismatch in nf_conntrack_reasm

WARNING: net/ipv6/netfilter/nf_defrag_ipv6.o(.text+0xe0): Section mismatch in
reference from the function nf_ct_net_init() to the function
.init.text:nf_ct_frag6_sysctl_register()
The function nf_ct_net_init() references the function
__init nf_ct_frag6_sysctl_register().

In case nf_conntrack_ipv6 is compiled as a module, nf_ct_net_init could be
called after the init code and data are unloaded. Therefore remove the
"__net_init" annotation from nf_ct_frag6_sysctl_register().

Signed-off-by: Hein Tibosch <hein_tibosch@yahoo.es>
Acked-by: Cong Wang <amwang@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>