[NETFILTER]: nf_conntrack_ipv6: fix sparse warnings

  CHECK   net/ipv6/netfilter/nf_conntrack_reasm.c
  net/ipv6/netfilter/nf_conntrack_reasm.c:77:18: warning: symbol 'nf_ct_ipv6_sysctl_table' was not declared. Should it be static?
  net/ipv6/netfilter/nf_conntrack_reasm.c:586:16: warning: symbol 'nf_ct_frag6_gather' was not declared. Should it be static?
  net/ipv6/netfilter/nf_conntrack_reasm.c:662:6: warning: symbol 'nf_ct_frag6_output' was not declared. Should it be static?
  net/ipv6/netfilter/nf_conntrack_reasm.c:683:5: warning: symbol 'nf_ct_frag6_kfree_frags' was not declared. Should it be static?
  net/ipv6/netfilter/nf_conntrack_reasm.c:698:5: warning: symbol 'nf_ct_frag6_init' was not declared. Should it be static?
  net/ipv6/netfilter/nf_conntrack_reasm.c:717:6: warning: symbol 'nf_ct_frag6_cleanup' was not declared. Should it be static?

Based on patch by Stephen Hemminger with suggestions by Yasuyuki KOZAKAI.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>