Staging: dst: crypto processing.

DST may fully encrypt the data channel in case of untrusted channel and implement
strong checksum of the transferred data. It is possible to configure algorithms
and crypto keys, they should match on both sides of the network channel.
Crypto processing does not introduce noticeble performance overhead, since DST
uses configurable pool of threads to perform crypto processing.

This patch introduces crypto processing helpers and crypto engine initialization:
glueing with the crypto layer, allocation and initialization of the crypto
processing thread pool, allocation of the cached pages, which are used to temporary
encrypt data into, since it is forbidden to encrypt data in-place, since pages
are used by the higher layers.

Signed-off-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>