ZTS: Count CKSUM for all vdevs in verify_pool

The verify_pool function should detect checksum errors on any vdev, but
it was only checking at the root of the pool.

Accumulate the errors for all vdevs to obtain the correct count.

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10271