btrfs: unify handling of return values of btrfs_insert_empty_items()

The error values returned by btrfs_insert_empty_items() are following
the common patter of 0/-errno, but some callers check for a value > 0,
which can't happen. Document that and update calls to not expect
positive values.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>