ipvs: Cache ipvs in ip_vs_in_icmp and ip_vs_in_icmp_v6

Storte the value of net_ipvs in a variable named ipvs so that when
there are more users struct netns_ipvs in ip_vs_in_cmp and
ip_vs_in_icmp_v6 they won't need to compute the value again.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>