tc/skbmod: Remove misinformation about the swap action

tc/skbmod: Remove misinformation about the swap action

Currently man 8 tc-skbmod says that "...the swap action will occur after
any smac/dmac substitutions are executed, if they are present."

This is false.  In fact, trying to "set" and "swap" in a single skbmod
command causes the "set" part to be completely ignored.  As an example:

	$ tc filter add dev eth0 parent 1: protocol ip prio 10 \
		matchall action skbmod \
        	set dmac AA:AA:AA:AA:AA:AA smac BB:BB:BB:BB:BB:BB \
        	swap mac

The above command simply does a "swap", without setting DMAC or SMAC to
AA's or BB's.  The root cause of this is in the kernel, see
net/sched/act_skbmod.c:tcf_skbmod_init():

	parm = nla_data(tb[TCA_SKBMOD_PARMS]);
	index = parm->index;
	if (parm->flags & SKBMOD_F_SWAPMAC)
		lflags = SKBMOD_F_SWAPMAC;
		^^^^^^^^^^^^^^^^^^^^^^^^^^

Doing a "=" instead of "|=" clears all other "set" flags when doing a
"swap".  Discourage using "set" and "swap" in the same command by
documenting it as undefined behavior, and update the "SYNOPSIS" section
as well as tc -help text accordingly.

If one really needs to e.g. "set" DMAC to all AA's then "swap" DMAC and
SMAC, one should do two separate commands and "pipe" them together.

Reviewed-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Peilin Ye <peilin.ye@bytedance.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>