fix directUpdate buffer pointer issue.

inside mapWithOffset already add the mBufferOffset

Signed-off-by: Hailin Zhang<hailinzhang@google.com>

Bug: b/207714894
Change-Id: Ia400bccbef1abc756cd8155e93a775338a30e8b9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3325025
Reviewed-by: Lingfeng Yang <lfy@google.com>
Reviewed-by: Charlie Lao <cclao@google.com>
Commit-Queue: Hailin Zhang <hailinzhang@google.com>

src/libANGLE/renderer/vulkan/BufferVk.cpp

@@ -1046,8 +1046,8 @@ angle::Result BufferVk::acquireAndUpdate(ContextVk *contextVk,
             ANGLE_TRY(
                 src->mapWithOffset(contextVk, &mapPointer, static_cast<size_t>(mBufferOffset)));
             ASSERT(mapPointer);
-            srcMapPtrBeforeSubData = mapPointer + mBufferOffset;
-            srcMapPtrAfterSubData  = mapPointer + mBufferOffset + offsetAfterSubdata;
+            srcMapPtrBeforeSubData = mapPointer;
+            srcMapPtrAfterSubData  = mapPointer + offsetAfterSubdata;
         }
     }