btrfs: uapi/linux/btrfs_tree.h, use __u8 and __u64

u8 and u64 aren't exported to userspace, while __u8 and __u64 are.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>