netfilter: nf_tables_offload: undo updates if transaction fails

The nft_flow_rule_offload_commit() function might fail after several
successful commands, thus, leaving the hardware filtering policy in
inconsistent state.

This patch adds nft_flow_rule_offload_abort() function which undoes the
updates that have been already processed if one command in this
transaction fails. Hence, the hardware ruleset is left as it was before
this aborted transaction.

The deletion path needs to create the flow_rule object too, in case that
an existing rule needs to be re-added from the abort path.

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