btrfs: Use explicit round_down macro in btrfs resize ioctl handler

No functional changes, just make the code more self-explanatory.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>