des: add possibility to calculate DES-CBC-MAC with small buffer

This patch adds the possibility to calculate the DES-CBC-MAC of a
source buffer (i.e. the last block of the buffer encrypted in CBC
mode) without having to allocate a destination buffer that is as
long as the complete source buffer, but instead only 8 bytes
for the MAC.

Signed-off-by: David Goldwich <david.goldwich@gmail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>