netfilter: ipvs: use NFPROTO values for NF_HOOK invocation

Semantic patch:
// <smpl>
@@
@@
 IP_VS_XMIT(
-PF_INET6,
+NFPROTO_IPV6,
 ...)

@@
@@
 IP_VS_XMIT(
-PF_INET,
+NFPROTO_IPV4,
 ...)
// </smpl>

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>