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

Here is the test result after conversion.

# ./gre_gso.sh
TEST: GREv6/v4 - copy file w/ TSO [ OK ]
TEST: GREv6/v4 - copy file w/ GSO [ OK ]
TEST: GREv6/v6 - copy file w/ TSO [ OK ]
TEST: GREv6/v6 - copy file w/ GSO [ OK ]

Tests passed: 4
Tests failed: 0

Acked-by: David Ahern <[e-mail]>
Signed-off-by: Hangbin Liu <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

tools/testing/selftests/net/gre_gso.sh

···
 # SPDX-License-Identifier: GPL-2.0
 
 # This test is for checking GRE GSO.
-
+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="gre_gso"
···
 VERBOSE=0
 PAUSE_ON_FAIL=no
 PAUSE=no
-IP="ip -netns ns1"
-NS_EXEC="ip netns exec ns1"
 TMPFILE=`mktemp`
 PID=
 
···
 setup()
 {
 	set -e
-	ip netns add ns1
-	ip netns set ns1 auto
-	$IP link set dev lo up
+	setup_ns ns1
+	IP="ip -netns $ns1"
+	NS_EXEC="ip netns exec $ns1"
 
 	ip link add veth0 type veth peer name veth1
 	ip link set veth0 up
-	ip link set veth1 netns ns1
+	ip link set veth1 netns $ns1
 	$IP link set veth1 name veth0
 	$IP link set veth0 up
 
···
 	[ -n "$PID" ] && kill $PID
 	ip link del dev gre1 &> /dev/null
 	ip link del dev veth0 &> /dev/null
-	ip netns del ns1
+	cleanup_ns $ns1
 }
 
 get_linklocal()
···
 	setup
 
 	a1=$(get_linklocal veth0)
-	a2=$(get_linklocal veth0 ns1)
+	a2=$(get_linklocal veth0 $ns1)
 
 	gre_create_tun $a1 $a2