netfilter: nf_tables: always initialize flowtable hook list in transaction

The hook list is used if nft_trans_flowtable_update(trans) == true. However,
initialize this list for other cases for safety reasons.

Fixes: 78d9f48f ("netfilter: nf_tables: add devices to existing flowtable")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>