netfilter: nf_tables: merge rt expression into nft core

before:
   text    data     bss     dec     hex filename
   2657     844       0    3501     dad net/netfilter/nft_rt.ko
 100826    2240     401  103467   1942b net/netfilter/nf_tables.ko
after:
   2657     844       0    3501     dad net/netfilter/nft_rt.ko
 102456    2316     401  105173   19ad5 net/netfilter/nf_tables.ko

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