selftests: rtnetlink: print device info on preferred_lft test failure

Even with slowwait used to avoid system sleep in the preferred_lft test,
failures can still occur after long runtimes.

Print the device address info when the test fails to provide better
troubleshooting data.

Signed-off-by: Hangbin Liu <[email protected]>
Signed-off-by: NipaLocal <nipa@local>

Author: liuhangbin

tools/testing/selftests/net/rtnetlink.sh

@@ -313,6 +313,8 @@ kci_test_addrlft()
 
 	slowwait 5 check_addr_not_exist "$devdummy" "10.23.11."
 	if [ $? -eq 1 ]; then
+		# troubleshoot the reason for our failure
+		run_cmd ip addr show dev "$devdummy"
 		check_err 1
 		end_test "FAIL: preferred_lft addresses remaining"
 		return