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 openzfs#10271

tests/zfs-tests/include/libtest.shlib

@@ -2029,7 +2029,12 @@ function verify_pool
 	log_must zpool scrub $pool
 	log_must wait_scrubbed $pool
 
-	cksum=$(zpool status $pool | awk 'L{print $NF;L=0} /CKSUM$/{L=1}')
+	typeset -i cksum=$(zpool status $pool | awk '
+	    !NF { isvdev = 0 }
+	    isvdev { errors += $NF }
+	    /CKSUM$/ { isvdev = 1 }
+	    END { print errors }
+	')
 	if [[ $cksum != 0 ]]; then
 		log_must zpool status -v
 	        log_fail "Unexpected CKSUM errors found on $pool ($cksum)"