Replaced the use of a Variable Length Array In Struct (VLAIS) with a
C99 compliant equivalent. This is the original VLAIS struct.

struct {
	struct aead_request     req;
	u8                      priv[crypto_aead_reqsize(tfm)];
} aead_req;

This patch instead allocates the appropriate amount of memory using a
char array.

The new code can be compiled with both gcc and clang.
Signed-off-by: Jan-Simon Möller <dl9pf@gmx.de>
Signed-off-by: Behan Webster <behanw@converseincode.com>
[small style cleanups]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>