kselftest: rtnetlink.sh: use grep_fail when expecting the cmd fail

run_cmd_grep_fail should be used when expecting the cmd fail, or the ret
will be set to 1, and the total test return 1 when exiting. This would cause
the result report to fail if run via run_kselftest.sh.

Before fix:
 # ./rtnetlink.sh -t kci_test_addrlft
 PASS: preferred_lft addresses have expired
 # echo $?
 1

After fix:
 # ./rtnetlink.sh -t kci_test_addrlft
 PASS: preferred_lft addresses have expired
 # echo $?
 0

Fixes: 9c2a19f ("kselftest: rtnetlink.sh: add verbose flag")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20231219065737.1725120-1-liuhangbin@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

tools/testing/selftests/net/rtnetlink.sh

@@ -297,7 +297,7 @@ kci_test_addrlft()
 	done
 
 	sleep 5
-	run_cmd_grep "10.23.11." ip addr show dev "$devdummy"
+	run_cmd_grep_fail "10.23.11." ip addr show dev "$devdummy"
 	if [ $? -eq 0 ]; then
 		check_err 1
 		end_test "FAIL: preferred_lft addresses remaining"