btrfs: create sprout should rename fsid on the sysfs as well

Creating sprout will change the fsid of the mounted root.
do the same on the sysfs as well.

reproducer:
 mount /dev/sdb /btrfs (seed disk)
 btrfs dev add /dev/sdc /btrfs
 mount -o rw,remount /btrfs
 btrfs dev del /dev/sdb /btrfs
 mount /dev/sdb /btrfs

Error:
kobject_add_internal failed for fe350492-dc28-4051-a601-e017b17e6145 with -EEXIST, don't try to register things with the same name in the same directory.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/volumes.c

@@ -2154,6 +2154,7 @@ int btrfs_init_new_device(struct btrfs_root *root, char *device_path)
 	mutex_unlock(&root->fs_info->fs_devices->device_list_mutex);
 
 	if (seeding_dev) {
+		char fsid_buf[BTRFS_UUID_UNPARSED_SIZE];
 		ret = init_first_rw_device(trans, root, device);
 		if (ret) {
 			btrfs_abort_transaction(trans, root, ret);
@@ -2164,6 +2165,14 @@ int btrfs_init_new_device(struct btrfs_root *root, char *device_path)
 			btrfs_abort_transaction(trans, root, ret);
 			goto error_trans;
 		}
+
+		/* Sprouting would change fsid of the mounted root,
+		 * so rename the fsid on the sysfs
+		 */
+		snprintf(fsid_buf, BTRFS_UUID_UNPARSED_SIZE, "%pU",
+						root->fs_info->fsid);
+		if (kobject_rename(&root->fs_info->super_kobj, fsid_buf))
+			goto error_trans;
 	} else {
 		ret = btrfs_add_device(trans, root, device);
 		if (ret) {

include/uapi/linux/btrfs.h

@@ -38,6 +38,7 @@ struct btrfs_ioctl_vol_args {
 #define BTRFS_SUBVOL_QGROUP_INHERIT	(1ULL << 2)
 #define BTRFS_FSID_SIZE 16
 #define BTRFS_UUID_SIZE 16
+#define BTRFS_UUID_UNPARSED_SIZE	37
 
 #define BTRFS_QGROUP_INHERIT_SET_LIMITS	(1ULL << 0)