netfilter: nf_tables: merge route type into core

very little code, so it really doesn't make sense to have extra
modules or even a kconfig knob for this.

Merge them and make functionality available unconditionally.
The merge makes inet family route support trivial, so add it
as well here.

Before:
   text	   data	    bss	    dec	    hex	filename
    835	    832	      0	   1667	    683 nft_chain_route_ipv4.ko
    870	    832	      0	   1702	    6a6	nft_chain_route_ipv6.ko
 111568	   2556	    529	 114653	  1bfdd	nf_tables.ko

After:
   text	   data	    bss	    dec	    hex	filename
 113133	   2556	    529	 116218	  1c5fa	nf_tables.ko

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