ZTS: get_persistent_disk_name can return truncated names

Instead of using only the 3rd element return the entire string after 
the split to handle device names with dashes.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Vaibhav Bhanawat <vaibhav.bhanawat@delphix.com>
Closes openzfs#15567

tests/zfs-tests/include/blkdev.shlib

@@ -289,7 +289,7 @@ function get_persistent_disk_name #device
 	if is_linux; then
 		if is_real_device $device; then
 			udevadm info -q all -n $DEV_DSKDIR/$device \
-			    | awk '/disk\/by-id/ {print $2; exit}' | cut -d/ -f3
+			    | awk '/disk\/by-id/ {print $2; exit}' | cut -d/ -f3-
 		elif is_mpath_device $device; then
 			udevadm info -q all -n $DEV_DSKDIR/$device \
 			    | awk '/disk\/by-id\/dm-uuid/ {print $2; exit}' \