netfilter: move handlers to net/ip_vs.h

Fix the following compilation warnings:
net/netfilter/ipvs/ip_vs_proto_tcp.c:147:1: warning: no previous prototype for 'tcp_snat_handler' [-Wmissing-prototypes]
  147 | tcp_snat_handler(struct sk_buff *skb, struct ip_vs_protocol *pp,
      | ^~~~~~~~~~~~~~~~
net/netfilter/ipvs/ip_vs_proto_udp.c:136:1: warning: no previous prototype for 'udp_snat_handler' [-Wmissing-prototypes]
  136 | udp_snat_handler(struct sk_buff *skb, struct ip_vs_protocol *pp,
      | ^~~~~~~~~~~~~~~~

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>