netfilter: nf_tables: remove nft_ctx_init_from_elemattr()

Replace nft_ctx_init_from_elemattr() by nft_table_lookup() and set up
the context structure right before it is really needed.

Moreover, nft_ctx_init_from_elemattr() is setting up the context
structure for codepaths where this is not really needed at all.

This helper function is also not helping to consolidate code, removing
it saves us 4 LoC.

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