s/VERIFY/VERIFY3S in vdev_checkpoint_sm_object

Using VERIFY3S allows to view the unexpected error value in the system
log.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Don Brady <don.brady@delphix.com>
Signed-off-by: Tim Chase <tim@chase2k.com>
Issue #7809
Closes #7818