Btrfs: Remove unnecessary placeholder in btrfs_err_code

"notused" is not necessary. Set 1 to the first entry is enough.

Signed-off-by: Takeuchi Satoru <takeuchi_satoru@jp.fujitsu.com
Cc: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
Reviewed-by: NDavid Sterba <dsterba@suse.cz>
Signed-off-by: NChris Mason <clm@fb.com>

include/uapi/linux/btrfs.h

@@ -495,8 +495,7 @@ struct btrfs_ioctl_send_args {
 
 /* Error codes as returned by the kernel */
 enum btrfs_err_code {
-	notused,
-	BTRFS_ERROR_DEV_RAID1_MIN_NOT_MET,
+	BTRFS_ERROR_DEV_RAID1_MIN_NOT_MET = 1,
 	BTRFS_ERROR_DEV_RAID10_MIN_NOT_MET,
 	BTRFS_ERROR_DEV_RAID5_MIN_NOT_MET,
 	BTRFS_ERROR_DEV_RAID6_MIN_NOT_MET,