selftests: drv-net: so_txtime: fix qdisc replace with handle

The blamed commit updated a tc replace command by adding a handle.

  -    tc(f"qdisc replace dev {ifname} root {qdisc} {optargs}")
  +    tc(f"qdisc replace dev {ifname} root handle 1: {qdisc} {optargs}")

This breaks the test if the root qdisc already has that handle and is of
different kind, with

  "Invalid qdisc name: must match existing qdisc."

If no handle is asked, or the kind differs, tc replace removes the old
qdisc and grafts a new one.

If a handle is asked and exists, tc replace tries to change the qdisc
in place, for which the kind must be the same.

It does not trigger in all setups, like netdevsim or debian 13, which
do not have root handle 1:. But it is a common root handle.

Solve the bug by first deleting the existing root qdisc if one exists.

Wrap that command in a try block, because it will fail for default
qdiscs with handle 0: with

  "Error: Cannot delete qdisc with handle of zero."

Reported-by: Jakub Kicinski <kuba@kernel.org>
Closes: https://lore.kernel.org/netdev/20260810183118.32d5c06a@kernel.org/
Fixes: ef3d6cca02 ("selftests: drv-net: so_txtime: only send test traffic to sch_etf")
Signed-off-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260811182856.2702163-1-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/drivers/net/so_txtime.py

@@ -12,6 +12,7 @@ import time
 from lib.py import ksft_exit, ksft_run, ksft_variants
 from lib.py import KsftNamedVariant, KsftSkipEx
 from lib.py import NetDrvEpEnv, bkg, cmd, defer, tc
+from lib.py import CmdExitFailure
 
 
 def test_so_txtime(cfg, clockid, ipver, args_tx, args_rx, expect_success):
@@ -45,6 +46,10 @@ def _qdisc_setup(ifname, qdisc, optargs=""):
     """
     orig = tc(f"qdisc show dev {ifname} root", json=True)[0].get("kind", None)
     defer(tc, f"qdisc replace dev {ifname} root {orig}")
+    try:
+        tc(f"qdisc del dev {ifname} root")
+    except CmdExitFailure:
+        pass
     tc(f"qdisc replace dev {ifname} root handle 1: {qdisc} {optargs}")