fiemap_writer: enforce filesystem optimal block size alignment

... instead of using the block size from the underlying block device.

Bug: 121211685
Test: fiemap_writer_test /dev/block/sda21 (on blueline)

Change-Id: I6038d3f06505a397280b0482e33814b4883be9a2
Signed-off-by: Sandeep Patil <sspatil@google.com>