netfilter: nf_tables: make meta expression builtin

size net/netfilter/nft_meta.ko
   text    data     bss     dec     hex filename
   5826     936       1    6763    1a6b net/netfilter/nft_meta.ko
  96407    2064     400   98871   18237 net/netfilter/nf_tables.ko

after:
 100826    2240     401  103467   1942b net/netfilter/nf_tables.ko

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