selftests: net: devlink_port_split.py: skip the test if no devlink device

When there is no devlink device, the following command will return:
  $ devlink -j dev show
  {dev:{}}

This will cause IndexError when trying to access the first element
in dev of this json dataset. Use the kselftest framework skip code
to skip this test in this case.

Example output with this change:
  # selftests: net: devlink_port_split.py
  # no devlink device was found, test skipped
  ok 7 selftests: net: devlink_port_split.py # SKIP

Link: https://bugs.launchpad.net/bugs/1928889
Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/devlink_port_split.py

@@ -18,6 +18,8 @@ import sys
 #
 
 
+# Kselftest framework requirement - SKIP code is 4
+KSFT_SKIP=4
 Port = collections.namedtuple('Port', 'bus_info name')
 
 
@@ -239,7 +241,11 @@ def main(cmdline=None):
         assert stderr == ""
 
         devs = json.loads(stdout)['dev']
+        if devs:
             dev = list(devs.keys())[0]
+        else:
+            print("no devlink device was found, test skipped")
+            sys.exit(KSFT_SKIP)
 
     cmd = "devlink dev show %s" % dev
     stdout, stderr = run_command(cmd)