libfiemap_writer: Calculate FIBMAP blocks correctly.

FIBMAP blocks are returned in FIGETBSZ units, which means the number of
blocks also needs to be determined by FIGETBSZ. Using the stat blocksize
is incorrect. On VFAT, FIGETBSZ returns 512 whereas st_blksize is 32768.

Bug: 126230649
Test: fiemap_writer_test gtest
Change-Id: Id0a667936ff9c0b60b1e8f72920cf62ceece1657