[vim3] fix `getvar partition-size:userdata`

We partially support the "userdata" partition by allowing erase to be a
no-op for the `fastboot -w` flag, but it doesn't actually exist on disk
so querying `partition-size:userdata` hit a buggy code path.

This code path is incorrect anyway, when a partition cannot be found we
need to return a fastboot FAIL message, but the code was printing an
error message inside the OKAY reponse instead which tricks the caller
into thinking the contents should be a partition size.

Bug: b/327961098
Change-Id: I46403152efcc12c617f5811674f438d4dab59323
Reviewed-on: https://turquoise-internal-review.googlesource.com/c/third_party/u-boot/+/815490
Reviewed-by: Yecheng Zhao <zyecheng@google.com>
GitOrigin-RevId: 642f58a81710552dd0d06e650e6a1bc330a6c4fd
diff --git a/drivers/usb/gadget/f_fastboot.c b/drivers/usb/gadget/f_fastboot.c
index 6fb9ba8..c14bab9 100644
--- a/drivers/usb/gadget/f_fastboot.c
+++ b/drivers/usb/gadget/f_fastboot.c
@@ -921,7 +921,9 @@
 					}
 				} else {
 					FB_DBG("find_mmc_partition_by_name fail\n");
-					sprintf(str_num, "get fail");
+					fastboot_tx_write_str(
+						"FAILpartition does not exist");
+					return;
 				}
 			}
 			strncat(response, str_num, chars_left);