netfilter: nf_tables: merge exthdr expression into nft core

before:
   text    data     bss     dec     hex filename
   5056     844       0    5900    170c net/netfilter/nft_exthdr.ko
 102456    2316     401  105173   19ad5 net/netfilter/nf_tables.ko

after:
 106410    2392     401  109203   1aa93 net/netfilter/nf_tables.ko

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