btrfs: add info when mount fails due to stale replace target

If the replace target device reappears after the suspended replace is
cancelled, it blocks the mount operation as it can't find the matching
replace-item in the metadata. As shown below,

   BTRFS error (device sda5): replace devid present without an active replace item

To overcome this situation, the user can run the command

   btrfs device scan --forget <replace target device>

and try the mount command again. And also, to avoid repeating the issue,
superblock on the devid=0 must be wiped.

   wipefs -a device-path-to-devid=0.

This patch adds some info when this situation occurs.
Reported-by: Samuel Greiner <samuel@balkonien.org>
Link: https://lore.kernel.org/linux-btrfs/b4f62b10-b295-26ea-71f9-9a5c9299d42c@balkonien.org/T/
CC: stable@vger.kernel.org # 5.0+
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/dev-replace.c

@@ -165,7 +165,7 @@ int btrfs_init_dev_replace(struct btrfs_fs_info *fs_info)
 		 */
 		if (btrfs_find_device(fs_info->fs_devices, &args)) {
 			btrfs_err(fs_info,
-			"replace devid present without an active replace item");
+"replace without active item, run 'device scan --forget' on the target device");
 			ret = -EUCLEAN;
 		} else {
 			dev_replace->srcdev = NULL;