[writeboost] add sector-aligned slab pool for I/O buffer

I/O buffer is required to be sector-aligned.
Using kmalloc may results in badly aligned buffer which leads to "Redzone written" error.

- This commit adds slab pools for 1-sector/8-sector buffer.
- check_buffer_alignment() is added to check if the buffer is correctly
  aligned.

Other buffers will be modified in later commits

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
Merge pull request #8 from akiradeveloper/feature/add-aligned-pool

[writeboost] add sector-aligned slab pool for I/O buffer