[writeboost] Don't pass __GFP_ZERO to mempool_alloc()

It's not allowed to pass __GFP_ZERO to mempool_alloc().
Without this fix, it warns at initialization.

Signed-off-by: Akira Hayakawa <ruby.wktk@gmail.com>
Merge pull request #21 from akiradeveloper/feature/fix-mempool-alloc-gfp-zero

[writeboost] Don't pass __GFP_ZERO to mempool_alloc()