crypto: ahash - ensure statesize is non-zero

Unlike shash algorithms, ahash drivers must implement export
and import as their descriptors may contain hardware state and
cannot be exported as is.  Unfortunately some ahash drivers did
not provide them and end up causing crashes with algif_hash.

This patch adds a check to prevent these drivers from registering
ahash algorithms until they are fixed.

Cc: stable@vger.kernel.org
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>