Btrfs: consistent usage of types in balance_args

The btrfs_balance_args are only used for the balance ioctl, so use __u
instead of __le here for consistency. The __le usage was introduced in
bc309467 and dee32d0a and was probably a result of
copy/pasting when the code was written.

The usage of __le did not break anything, but it's unnecessary. Also,
this change makes the code less confusing for the careful reader.

Signed-off-by: Hans van Kranenburg <hans.van.kranenburg@mendix.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>