KEYS: encrypted: Use SHA-256 library instead of crypto_shash

KEYS: encrypted: Use SHA-256 library instead of crypto_shash

Instead of the "sha256" crypto_shash, just use sha256().  Similarly,
instead of the "hmac(sha256)" crypto_shash, just use
hmac_sha256_usingrawkey().  This is simpler and faster.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>