This patches reworks the way the algorithm type is set in the context,
by using the fact that the decryption algorithms are just a combination
of the algorithm encryption type and CONTEXT_CONTROL_TYPE_NULL_IN.

This will help having simpler code when adding the AEAD support, to
avoid ending up with an endless switch case block.
Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>