Btrfs: return failure if btrfs_dev_replace_finishing() failed

device replace could fail due to another running scrub process or any
other errors btrfs_scrub_dev() may hit, but this failure doesn't get
returned to userspace.

The following steps could reproduce this issue

	mkfs -t btrfs -f /dev/sdb1 /dev/sdb2
	mount /dev/sdb1 /mnt/btrfs
	while true; do btrfs scrub start -B /mnt/btrfs >/dev/null 2>&1; done &
	btrfs replace start -Bf /dev/sdb2 /dev/sdb3 /mnt/btrfs
	# if this replace succeeded, do the following and repeat until
	# you see this log in dmesg
	# BTRFS: btrfs_scrub_dev(/dev/sdb2, 2, /dev/sdb3) failed -115
	#btrfs replace start -Bf /dev/sdb3 /dev/sdb2 /mnt/btrfs

	# once you see the error log in dmesg, check return value of
	# replace
	echo $?

Introduce a new dev replace result

BTRFS_IOCTL_DEV_REPLACE_RESULT_SCRUB_INPROGRESS

to catch -EINPROGRESS explicitly and return other errors directly to
userspace.
Signed-off-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/dev-replace.c

@@ -422,9 +422,15 @@ int btrfs_dev_replace_start(struct btrfs_root *root,
 			      &dev_replace->scrub_progress, 0, 1);
 
 	ret = btrfs_dev_replace_finishing(root->fs_info, ret);
-	WARN_ON(ret);
+	/* don't warn if EINPROGRESS, someone else might be running scrub */
+	if (ret == -EINPROGRESS) {
+		args->result = BTRFS_IOCTL_DEV_REPLACE_RESULT_SCRUB_INPROGRESS;
+		ret = 0;
+	} else {
+		WARN_ON(ret);
+	}
 
-	return 0;
+	return ret;
 
 leave:
 	dev_replace->srcdev = NULL;
@@ -542,7 +548,7 @@ static int btrfs_dev_replace_finishing(struct btrfs_fs_info *fs_info,
 			btrfs_destroy_dev_replace_tgtdev(fs_info, tgt_device);
 		mutex_unlock(&dev_replace->lock_finishing_cancel_unmount);
 
-		return 0;
+		return scrub_ret;
 	}
 
 	printk_in_rcu(KERN_INFO

include/uapi/linux/btrfs.h

@@ -157,6 +157,7 @@ struct btrfs_ioctl_dev_replace_status_params {
 #define BTRFS_IOCTL_DEV_REPLACE_RESULT_NO_ERROR			0
 #define BTRFS_IOCTL_DEV_REPLACE_RESULT_NOT_STARTED		1
 #define BTRFS_IOCTL_DEV_REPLACE_RESULT_ALREADY_STARTED		2
+#define BTRFS_IOCTL_DEV_REPLACE_RESULT_SCRUB_INPROGRESS		3
 struct btrfs_ioctl_dev_replace_args {
 	__u64 cmd;	/* in */
 	__u64 result;	/* out */