[writeboost] use vmalloc in zeroing_full_superblock

allocating with kmalloc may results in unaligned buffer.

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
[writeboost] use 1 sector mempool in format_superblock_header

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
[writeboost] add rambuf_cachep to allocate aligned RAM buffers

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
[writeboost] use rambuf_cachep in flush_plog

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
Merge pull request #9 from akiradeveloper/feature/add-rambuf-slab-v2

Feature/add rambuf slab