selftests: rtnetlink: fix addrlft test flakiness on power-saving systems

Jakub reported that the rtnetlink test for the preferred lifetime of an
address has become quite flaky. The issue started appearing around the 6.16
merge window in May, and the test fails with:

    FAIL: preferred_lft addresses remaining

The flakiness might be related to power-saving behavior, as address
expiration is handled by a "power-efficient" workqueue.

To address this, use slowwait to check more frequently whether the address
still exists. This reduces the likelihood of the system entering a low-power
state during the test, improving reliability.

Reported-by: Jakub Kicinski <[email protected]>
Signed-off-by: Hangbin Liu <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: liuhangbin

tools/testing/selftests/net/rtnetlink.sh

@@ -291,16 +291,26 @@ kci_test_route_get()
 	end_test "PASS: route get"
 }
 
+check_addr_not_exist()
+{
+	dev=$1
+	addr=$2
+	if ip addr show dev $dev | grep -q $addr; then
+		return 1
+	else
+		return 0
+	fi
+}
+
 kci_test_addrlft()
 {
 	for i in $(seq 10 100) ;do
 		lft=$(((RANDOM%3) + 1))
 		run_cmd ip addr add 10.23.11.$i/32 dev "$devdummy" preferred_lft $lft valid_lft $((lft+1))
 	done
 
-	sleep 5
-	run_cmd_grep_fail "10.23.11." ip addr show dev "$devdummy"
-	if [ $? -eq 0 ]; then
+	slowwait 5 check_addr_not_exist "$devdummy" "10.23.11."
+	if [ $? -eq 1 ]; then
 		check_err 1
 		end_test "FAIL: preferred_lft addresses remaining"
 		return