btrfs: avoid transaction commit on error in del_balance_item()

btrfs: avoid transaction commit on error in del_balance_item()

There's no point in committing the transaction if we failed to delete the
item, since we haven't done anything before. Also stop using two variables
for tracking the return value and use only 'ret'.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@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>