net: mscc: ocelot: fix VCAP IS2 filters matching on both lookups

The VCAP IS2 TCAM is looked up twice per packet, and each filter can be
configured to only match during the first, second lookup, or both, or
none.

The blamed commit wrote the code for making VCAP IS2 filters match only
on the given lookup. But right below that code, there was another line
that explicitly made the lookup a "don't care", and this is overwriting
the lookup we've selected. So the code had no effect.

Some of the more noticeable effects of having filters match on both
lookups:

- in "tc -s filter show dev swp0 ingress", we see each packet matching a
  VCAP IS2 filter counted twice. This throws off scripts such as
  tools/testing/selftests/net/forwarding/tc_actions.sh and makes them
  fail.

- a "tc-drop" action offloaded to VCAP IS2 needs a policer as well,
  because once the CPU port becomes a member of the destination port
  mask of a packet, nothing removes it, not even a PERMIT/DENY mask mode
  with a port mask of 0. But VCAP IS2 rules with the POLICE_ENA bit in
  the action vector can only appear in the first lookup. What happens
  when a filter matches both lookups is that the action vector is
  combined, and this makes the POLICE_ENA bit ineffective, since the
  last lookup in which it has appeared is the second one. In other
  words, "tc-drop" actions do not drop packets for the CPU port, dropped
  packets are still seen by software unless there was an FDB entry that
  directed those packets to some other place different from the CPU.

The last bit used to work, because in the initial commit b5962294
("net: mscc: ocelot: Add support for tcam"), we were writing the FIRST
field of the VCAP IS2 half key with a 1, not with a "don't care".
The change to "don't care" was made inadvertently by me in commit
c1c3993e ("net: mscc: ocelot: generalize existing code for VCAP"),
which I just realized, and which needs a separate fix from this one,
for "stable" kernels that lack the commit blamed below.

Fixes: 226e9cd8 ("net: mscc: ocelot: only install TCAM entries into a specific lookup and PAG")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/mscc/ocelot_vcap.c

@@ -374,7 +374,6 @@ static void is2_entry_set(struct ocelot *ocelot, int ix,
 			 OCELOT_VCAP_BIT_0);
 	vcap_key_set(vcap, &data, VCAP_IS2_HK_IGR_PORT_MASK, 0,
 		     ~filter->ingress_port_mask);
-	vcap_key_bit_set(vcap, &data, VCAP_IS2_HK_FIRST, OCELOT_VCAP_BIT_ANY);
 	vcap_key_bit_set(vcap, &data, VCAP_IS2_HK_HOST_MATCH,
 			 OCELOT_VCAP_BIT_ANY);
 	vcap_key_bit_set(vcap, &data, VCAP_IS2_HK_L2_MC, filter->dmac_mc);