In preparation to enabling -Wvla, remove VLAs and replace them with
fixed-length arrays instead.

The arrays fixed here, using the number of constant sections, aren't
really VLAs, but they appear so to the compiler. Replace the array sizes
with a pre-processor-level constant instead using ARRAY_SIZE.

This was prompted by https://lkml.org/lkml/2018/3/7/621Signed-off-by: NStephen Kitt <steve@sk2.org>
Reviewed-by: NKees Cook <keescook@chromium.org>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>