[libfmq] Finish rolling zerocopy Bug: 367420574 Change-Id: Ib0fff4a36edc1daf74eabefb3c5ce11b958cfc8b Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/android.googlesource.com/platform/system/libfmq/+/1121534 Reviewed-by: Adam Barth <abarth@google.com>