[PATCH] btrfs: oops in device_list_add()

[Al Viro <viro@xxxxxxxxxxxxxxxxxx>]

	alloc_fs_devices() can return ERR_PTR(-ENOMEM), so
dereferencing its result before the check for IS_ERR() is
a bad idea...

Fixes: d1a63002829a4
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
---
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 85149f27644d..72adc5643bde 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -955,11 +955,11 @@ static noinline struct btrfs_device *device_list_add(const char *path,
 		else
 			fs_devices = alloc_fs_devices(disk_super->fsid, NULL);
 
-		fs_devices->fsid_change = fsid_change_in_progress;
-
 		if (IS_ERR(fs_devices))
 			return ERR_CAST(fs_devices);
 
+		fs_devices->fsid_change = fsid_change_in_progress;
+
 		mutex_lock(&fs_devices->device_list_mutex);
 		list_add(&fs_devices->fs_list, &fs_uuids);