IPVS: Move IPVS to net/netfilter/ipvs

Since IPVS now has partial IPv6 support, this patch moves IPVS from
net/ipv4/ipvs to net/netfilter/ipvs. It's a result of:

$ git mv net/ipv4/ipvs net/netfilter

and adapting the relevant Kconfigs/Makefiles to the new path.

Signed-off-by: Julius Volz <juliusv@google.com>
Signed-off-by: Simon Horman <horms@verge.net.au>