netfilter: nat: merge ipv4 and ipv6 masquerade functionality

Before:
   text	   data	    bss	    dec	    hex	filename
  13916	   1412	   4128	  19456	   4c00	nf_nat.ko
   4510	    968	      4	   5482	   156a	nf_nat_ipv4.ko
   5146	    944	      8	   6098	   17d2	nf_nat_ipv6.ko

After:
   text	   data	    bss	    dec	    hex	filename
  16566	   1576	   4136	  22278	   5706	nf_nat.ko
   3187	    844	      0	   4031	    fbf	nf_nat_ipv4.ko
   3598	    844	      0	   4442	   115a	nf_nat_ipv6.ko

... so no drastic changes in combined size.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>