hibernate: generate and compare digest in nocompress path

This patch generate digest of snapshot in save_image and compare digest
in load_image. It's hurt the performance of load_image a lot becuase need
force to sync-load snapshot data from block-io, I will try to fix it
later.
This patch will need adapt to RSA signature generation/verification
mechanism, and also add CONFIG and force parameter like kernel module
sign facility.

Signed-off-by: Lee, Chun-Yi <jlee@suse.com>