Btrfs: use _IOR for BTRFS_IOC_SUBVOL_GETFLAGS

We used the wrong ioctl macro for the getflags ioctl before.
As we don't have the set/getflags ioctls in the user space ioctl.h
at the moment, it's safe to fix it now.

Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Alexander Block <ablock84@googlemail.com>