Fix imprecise name of a fs-verity helper function

Root hash is only part of the fs-verity digest covers. Rename to avoid
confusion.

Bug: N/A
Test: m
Change-Id: I12c2a4ac57fb8772483473f951e97474ebd894c9
Merged-In: I12c2a4ac57fb8772483473f951e97474ebd894c9
2 files changed