btrfs: clarify btrfs_ioctl_get_dev_stats padding

In commit c11d2c23 ("Btrfs: add ioctl to get and reset the device
stats") the get_dev_stats ioctl was added.

Shortly thereafter, in commit b27f7c0c ("btrfs: join DEV_STATS
ioctls to one") , the flags field was added.  However, the calculation
for unused padding space was not updated, which also invalidated the
comment.

Clarify what happened to reduce confusion and wasted time for anyone
implementing this.

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>