async_pq: Remove VLA usage

In the quest to remove VLAs from the kernel[1], this adjusts the
allocation of coefs and blocks to use the existing maximum values
(with one new define, MAX_DISKS for coefs, and a reuse of the
existing NDISKS for blocks).

[1] https://lkml.org/lkml/2018/3/7/621



Signed-off-by: Kyle Spiers <ksspiers@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>