selftests/net: convert drop_monitor_tests.sh to run it in unique namespace

Here is the test result after conversion.

]# ./drop_monitor_tests.sh

Software drops test
TEST: Capturing active software drops [ OK ]
TEST: Capturing inactive software drops [ OK ]

Hardware drops test
TEST: Capturing active hardware drops [ OK ]
TEST: Capturing inactive hardware drops [ OK ]

Tests passed: 4
Tests failed: 0

Acked-by: David Ahern <[e-mail]>
Signed-off-by: Hangbin Liu <[e-mail]>
Signed-off-by: Paolo Abeni <[e-mail]>

tools/testing/selftests/net/drop_monitor_tests.sh

···
 # SPDX-License-Identifier: GPL-2.0
 
 # This test is for checking drop monitor functionality.
-
+source lib.sh
 ret=0
-# Kselftest framework requirement - SKIP code is 4.
-ksft_skip=4
 
 # all tests in this script. Can be overridden with -t option
 TESTS="
···
 	hw_drops
 "
 
-IP="ip -netns ns1"
-TC="tc -netns ns1"
-DEVLINK="devlink -N ns1"
-NS_EXEC="ip netns exec ns1"
 NETDEVSIM_PATH=/sys/bus/netdevsim/
 DEV_ADDR=1337
 DEV=netdevsim${DEV_ADDR}
···
 	modprobe netdevsim &> /dev/null
 
 	set -e
-	ip netns add ns1
+	setup_ns NS1
 	$IP link add dummy10 up type dummy
 
 	$NS_EXEC echo "$DEV_ADDR 1" > ${NETDEVSIM_PATH}/new_device
···
 cleanup()
 {
 	$NS_EXEC echo "$DEV_ADDR" > ${NETDEVSIM_PATH}/del_device
-	ip netns del ns1
+	cleanup_ns ${NS1}
 }
 
 sw_drops_test()
···
 	exit $ksft_skip
 fi
 
-# start clean
+# create netns first so we can get the namespace name
+setup_ns NS1
 cleanup &> /dev/null
+trap cleanup EXIT
+
+IP="ip -netns ${NS1}"
+TC="tc -netns ${NS1}"
+DEVLINK="devlink -N ${NS1}"
+NS_EXEC="ip netns exec ${NS1}"
 
 for t in $TESTS
 do