In ingress and clsact qdisc TCA_OPTIONS are ignored, since it's
parameterless. In tc, we add an empty addattr_l(... TCA_OPTIONS,
NULL, 0) to the netlink message nevertheless. This has the
side effect that when someone tries a 'tc qdisc replace' and
already an existing such qdisc is present, tc fails with
EINVAL here.

Reason is that in the kernel, this invokes qdisc_change() when
such requested qdisc is already present. When TCA_OPTIONS are
passed to modify parameters, it looks whether qdisc implements
.change() callback, and if not present (like in both cases here)
it returns with error. Rather than adding an empty stub to the
kernel that ignores TCA_OPTIONS again, just don't add TCA_OPTIONS
to the netlink message in the first place.

Before:

  # tc qdisc replace dev foo clsact    # first try
  # tc qdisc replace dev foo clsact    # second one
  RTNETLINK answers: Invalid argument

After:

  # tc qdisc replace dev foo clsact
  # tc qdisc replace dev foo clsact
  # tc qdisc replace dev foo clsact
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>