crypto: aead - Add aead_alg_instance

Now that type-safe init/exit functions exist, they often need
to access the underlying aead_instance.  So this patch adds the
helper aead_alg_instance to access aead_instance from a crypto_aead
object.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --cc include/crypto/internal/aead.h
Simple merge