fiemap_writer_test: add block truncation and sync for safety

There is a delay to get inode evicted whic requires to deallocate blocks used
by the previous test. Let's secure the pinned space as much as possible.

Bug: 371822983
Change-Id: I4cc69fc61e348511bb027ca0a9dd277fffb20f7f
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>