[PATCHv2] selftests: xfrm: put cleanup code into an exit trap

If the xfrm_policy.sh script gets terminated by any reason, the netns
namespace files created by the test will be left alone.

In this case a second attempt to run this test will fail with:
  # Cannot create namespace file "/run/netns/ns1": File exists

Move the netns cleanup code into an exit trap so that we can ensure
these files will be removed in the end.

Changes in V2:
  - Update commit message and patch title.

Signed-off-by: Po-Hsu Lin <redacted>
---
 tools/testing/selftests/net/xfrm_policy.sh | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/net/xfrm_policy.sh b/tools/testing/selftests/net/xfrm_policy.sh
index bdf450e..bb4632b 100755
--- a/tools/testing/selftests/net/xfrm_policy.sh
+++ b/tools/testing/selftests/net/xfrm_policy.sh

@@ -28,6 +28,11 @@ KEY_AES=0x0123456789abcdef0123456789012345
 SPI1=0x1
 SPI2=0x2
 
+cleanup() {
+    for i in 1 2 3 4;do ip netns del ns$i 2>/dev/null ;done
+}
+trap cleanup EXIT
+
 do_esp_policy() {
     local ns=$1
     local me=$2

@@ -481,6 +486,4 @@ check_hthresh_repeat "policies with repeated htresh change"
 
 check_random_order ns3 "policies inserted in random order"
 
-for i in 1 2 3 4;do ip netns del ns$i;done
-
 exit $ret

-- 
2.7.4