btrfs: Use __u64 in exported linux/btrfs.h.

This patch fixes this build error.

/usr/include/linux/btrfs.h:121:3: error: unknown type name ‘u64’
   u64 devid;
   ^~~

Fixes: 6b526ed7 ("btrfs: introduce device delete by devid")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Signed-off-by: David Sterba <dsterba@suse.com>

include/uapi/linux/btrfs.h

@@ -118,7 +118,7 @@ struct btrfs_ioctl_vol_args_v2 {
 	};
 	union {
 		char name[BTRFS_SUBVOL_NAME_MAX + 1];
-		u64 devid;
+		__u64 devid;
 	};
 };