libfstab_fuzzer: fuzz TransformFstabForDsu() and skip mount logic

These functions contain complex logic and parse user input. It shall be
valuable to fuzz these functions to prevent regression.

Bug: 254832225
Test: Build and run the fuzzer
Change-Id: I21099c3fc1c226f95a0f8f996bb751030e0c59bc
3 files changed