libfiemap_writer: Fix the progress bar on VFAT partitions.

We allocate VFAT file space by seeking and writing a single byte. It
turns out this is not fast enough to avoid invoking the progress
callback, so instead, we should seek in increments.

Bug: 126230649
Test: fiemap_writer_test gtest
Change-Id: I2b363af4ebde873411e8a3acd22ca68d175afe3f