netfilter: nf_tables: add userdata attributes to nft_table

Enables storing userdata for nft_table. Field udata points to user data
and udlen store its length.

Adds new attribute flag NFTA_TABLE_USERDATA

Signed-off-by: Jose M. Guisado Gomez <guigom@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>