netfilter: flowtable: remove ipv4/ipv6 modules

Just place the structs and registration in the inet module.
nf_flow_table_ipv6, nf_flow_table_ipv4 and nf_flow_table_inet share
same module dependencies: nf_flow_table, nf_tables.

before:
   text	   data	    bss	    dec	    hex	filename
   2278	   1480	      0	   3758	    eae	nf_flow_table_inet.ko
   1159	   1352	      0	   2511	    9cf	nf_flow_table_ipv6.ko
   1154	   1352	      0	   2506	    9ca	nf_flow_table_ipv4.ko

after:
   2369	   1672	      0	   4041	    fc9	nf_flow_table_inet.ko

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