[CRYPTO] hmac: Add crypto template implementation

This patch rewrites HMAC as a crypto template.  This means that HMAC is no
longer a hard-coded part of the API.  It's now a template that generates
standard digest algorithms like any other.

The old HMAC is preserved until all current users are converted.

The same structure can be used by other MACs such as AES-XCBC-MAC.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>