netfilter: nf_tables: relocate commit and abort routines in the source file

Move the commit and abort routines to the bottom of the source code
file. This change is required by the follow up patches that add the
set, chain and table transaction support.

This patch is just a cleanup to access several functions without
having to declare their prototypes.

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