Porting over some ICP code that was missed in #4760

When #4760 was merged tests were added to ensure that the new checksums
were working properly. However, some of the functionality for sha2
functions were not ported over, resulting in some Coverity defects and
code that would be unstable when needed in the future. This patch
simply ports over the missing code and fixes the defects in the
process.

Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Issue #4760
Closes #5251