It gives significant improvements ( ~+15%) on some modes.

These code has been adopted from OpenSSL project in collaboration
with the original author (Andy Polyakov <appro@openssl.org>).
Signed-off-by: NPaulo Flabiano Smorigo <pfsmorigo@linux.vnet.ibm.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>