Track the key path on the SuspenseBoundary

This is unfortunate that we need to add this to the render path in general
but this doesn't really cost anything extra in terms of allocations since
this object would live anyway so it's an extra field.

packages/react-server/src/ReactFizzServer.js

@@ -181,6 +181,7 @@ type SuspenseBoundary = {
   byteSize: number, // used to determine whether to inline children boundaries.
   fallbackAbortableTasks: Set<Task>, // used to cancel task on the fallback if the boundary completes or gets canceled.
   resources: BoundaryResources,
+  keyPath: KeyNode,
 };
 
 export type Task = {
@@ -385,6 +386,7 @@ function pingTask(request: Request, task: Task): void {
 function createSuspenseBoundary(
   request: Request,
   fallbackAbortableTasks: Set<Task>,
+  keyPath: KeyNode,
 ): SuspenseBoundary {
   return {
     id: UNINITIALIZED_SUSPENSE_BOUNDARY_ID,
@@ -397,6 +399,7 @@ function createSuspenseBoundary(
     fallbackAbortableTasks,
     errorDigest: null,
     resources: createBoundaryResources(),
+    keyPath,
   };
 }
 
@@ -590,7 +593,7 @@ function renderSuspenseBoundary(
   const content: ReactNodeList = props.children;
 
   const fallbackAbortSet: Set<Task> = new Set();
-  const newBoundary = createSuspenseBoundary(request, fallbackAbortSet);
+  const newBoundary = createSuspenseBoundary(request, fallbackAbortSet, task.keyPath);
   const insertionIndex = parentSegment.chunks.length;
   // The children of the boundary segment is actually the fallback.
   const boundarySegment = createPendingSegment(