Use dsl_scan_setup_check() to setup a scrub

Use dsl_scan_setup_check() to setup a scrub

When a rebuild completes it will automatically schedule a follow up
scrub to verify all of the block checksums.  Before setting up the
scrub execute the counterpart dsl_scan_setup_check() function to
confirm the scrub can be started.  Prior to this change we'd only
check vdev_rebuild_active() which isn't as comprehensive, and using
the check function keeps all of this logic in one place.

Reviewed-by: Mark Maybee <mark.maybee@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11849