btrfs: avoid transaction commit on error in insert_balance_item()

btrfs: avoid transaction commit on error in insert_balance_item()

There's no point in committing the transaction if we failed to insert the
balance item, since we haven't done anything else after we started/joined
the transaction. Also stop using two variables for tracking the return
value and use only 'ret'.

Reviewed-by: Daniel Vacek <neelx@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>