ima: provide double buffering for hash calculation

The asynchronous hash API allows initiating a hash calculation and
then performing other tasks, while waiting for the hash calculation
to complete.

This patch introduces usage of double buffering for simultaneous
hashing and reading of the next chunk of data from storage.

Changes in v3:
- better comments

Signed-off-by: Dmitry Kasatkin <d.kasatkin@samsung.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>