netfilter: nf_tables: deconstify table and chain in context structure

The new transaction infrastructure updates the family, table and chain
objects in the context structure, so let's deconstify them. While at it,
move the context structure initialization routine to the top of the
source file as it will be also used from the table and chain routines.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>