btrfs: update sprout seed pointer when seed fs is relinquished

We are not updating sprout fs seed pointer when all seed device
is replaced. This patch will check if all seed device has been
replaced and then update the sprout pointer accordingly.

Same reproducer as in the previous patch would apply here.
And notice that btrfs_close_device will check if seed fs is
present and spits out the error with out this patch.

int btrfs_close_devices(struct btrfs_fs_devices *fs_devices)
{
::
                seed_devices = fs_devices->seed;
::
        while (seed_devices) {
                fs_devices = seed_devices;
                seed_devices = fs_devices->seed;
                __btrfs_close_devices(fs_devices);
                free_fs_devices(fs_devices);
        }
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/volumes.c

@@ -1848,6 +1848,27 @@ void btrfs_rm_dev_replace_srcdev(struct btrfs_fs_info *fs_info,
 	}
 
 	call_rcu(&srcdev->rcu, free_device);
+
+	/*
+	 * unless fs_devices is seed fs, num_devices shouldn't go
+	 * zero
+	 */
+	BUG_ON(!fs_devices->num_devices && !fs_devices->seeding);
+
+	/* if this is no devs we rather delete the fs_devices */
+	if (!fs_devices->num_devices) {
+		struct btrfs_fs_devices *tmp_fs_devices;
+
+		tmp_fs_devices = fs_info->fs_devices;
+		while (tmp_fs_devices) {
+			if (tmp_fs_devices->seed == fs_devices) {
+				tmp_fs_devices->seed = fs_devices->seed;
+				break;
+			}
+			tmp_fs_devices = tmp_fs_devices->seed;
+		}
+		fs_devices->seed = NULL;
+	}
 }
 
 void btrfs_destroy_dev_replace_tgtdev(struct btrfs_fs_info *fs_info,