netfilter: nf_tables: fix error handling of rule replacement

In general, if a transaction object is added to the list successfully,
we can rely on the abort path to undo what we've done. This allows us to
simplify the error handling of the rule replacement path in
nf_tables_newrule().

This implicitly fixes an unnecessary removal of the old rule, which
needs to be left in place if we fail to replace.

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