crypto: LLVMLinux: Remove VLAIS usage from crypto/hmac.c

crypto: LLVMLinux: Remove VLAIS usage from crypto/hmac.c

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: Jan-Simon Möller <[email protected]>
Signed-off-by: Behan Webster <[email protected]>
Reviewed-by: Mark Charlebois <[email protected]>
Acked-by: Herbert Xu <[email protected]>
Cc: [email protected]