crypto: krb5 - Use SG miter instead of doing it by hand

The function crypto_shash_update_sg iterates through an SG by
hand.  It fails to handle corner cases such as SG entries longer
than a page.  Fix this by using the SG iterator.

Fixes: 348f5669 ("crypto/krb5: Implement the Kerberos5 rfc3961 get_mic and verify_mic")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>