Store the checksum in PostPolicyHandler so that we can return it on Get/Head (#20364)

cmd/bucket-handlers.go

@@ -1200,6 +1200,7 @@ func (api objectAPIHandlers) PostPolicyBucketHandler(w http.ResponseWriter, r *h
 		writeErrorResponseHeadersOnly(w, toAPIError(ctx, err))
 		return
 	}
+	opts.WantChecksum = checksum
 
 	fanOutOpts := fanOutOptions{Checksum: checksum}