ipvs: fix warning on unused variable

When CONFIG_IP_VS_IPV6 is not defined, build produced this warning:

net/netfilter/ipvs/ip_vs_ctl.c:899:6: warning: unused variable ‘ret’ [-Wunused-variable]
  int ret = 0;
      ^~~

Fix this by moving the declaration of 'ret' in the CONFIG_IP_VS_IPV6
section in the same function.

While at it, drop its unneeded initialisation.

Fixes: 098e13f5

 ("ipvs: fix dependency on nf_defrag_ipv6")
Reported-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>