[writeboost] fix: u64 cast places wrongly

u64 cast is needed to cast sector_t to u64 that is feeded into
cpu_to_le64. In this case the output of calc_cache_alignment()

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
Merge pull request #3 from akiradeveloper/feature/fix-u64-cast

[writeboost] fix: u64 cast places wrongly