netfilter: nf_tables: register hooks last when adding new chain/flowtable

Register hooks last when adding chain/flowtable to ensure that packets do
not walk over datastructure that is being released in the error path
without waiting for the rcu grace period.

Fixes: 91c7b38d ("netfilter: nf_tables: use new transaction infrastructure to handle chain")
Fixes: 3b49e2e9 ("netfilter: nf_tables: add flow table netlink frontend")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>