[NETFILTER]: nf_nat: add helpers for common NAT protocol operations

Add generic ->in_range and ->unique_tuple ops to avoid duplicating them
again and again for future NAT modules and save a few bytes of text:

net/ipv4/netfilter/nf_nat_proto_tcp.c:
  tcp_in_range     |  -62 (removed)
  tcp_unique_tuple | -259 # 271 -> 12, # inlines: 1 -> 0, size inlines: 7 -> 0
 2 functions changed, 321 bytes removed

net/ipv4/netfilter/nf_nat_proto_udp.c:
  udp_in_range     |  -62 (removed)
  udp_unique_tuple | -259 # 271 -> 12, # inlines: 1 -> 0, size inlines: 7 -> 0
 2 functions changed, 321 bytes removed

net/ipv4/netfilter/nf_nat_proto_gre.c:
  gre_in_range |  -62 (removed)
 1 function changed, 62 bytes removed

vmlinux:
 5 functions changed, 704 bytes removed

Signed-off-by: Patrick McHardy <kaber@trash.net>