This patch switches hmac over to the new init_tfm/exit_tfm interface
as opposed to cra_init/cra_exit.  This way the shash API can make
sure that descsize does not exceed the maximum.

This patch also adds the API helper shash_alg_instance.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>