Btrfs: Compile warning fixup in volume.c

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/volumes.c

@@ -547,7 +547,7 @@ int btrfs_add_device(struct btrfs_trans_handle *trans,
 	struct extent_buffer *leaf;
 	struct btrfs_key key;
 	unsigned long ptr;
-	u64 free_devid;
+	u64 free_devid = 0;
 
 	root = root->fs_info->chunk_root;