Replaced the use of a Variable Length Array In Struct (VLAIS) with a C99
compliant equivalent. This patch allocates the appropriate amount of memory
using a char array using the SHASH_DESC_ON_STACK macro.

The new code can be compiled with both gcc and clang.
Signed-off-by: NJan-Simon Möller <dl9pf@gmx.de>
Signed-off-by: NBehan Webster <behanw@converseincode.com>
Reviewed-by: NMark Charlebois <charlebm@gmail.com>
Acked-by: NHerbert Xu <herbert@gondor.apana.org.au>
Cc: pageexec@freemail.hu