crypto: aead - Add type-safe init/exit functions

As it stands the only non-type safe functions left in the new
AEAD interface are the cra_init/cra_exit functions.  It means
exposing the ugly __crypto_aead_cast to every AEAD implementor.

This patch adds type-safe init/exit functions to AEAD.  Existing
algorithms are unaffected while new implementations can simply
fill in these two instead of cra_init/cra_exit.

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