Calculate correct checksum.

To maintain parity with fs-verity, we need to calculate checksums
according to https://www.kernel.org/doc/html/latest/filesystems/fsverity.html#fs-verity-descriptor

Bug: 233217301
Fixes: 233217301
Test: atest ChecksumsTest
Change-Id: Ie0108f12ba988536a1d2fd9dddf1614681026f7c