Fletcher4: Incremental updates and ctx calculation

Fixes ABI issues with fletcher4 code, adds support for
incremental updates, and adds ztest method for testing.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Gvozden Neskovic <neskovic@gmail.com>
Closes #5164