selftests: mlxsw: Add tc sample tests

Test that packets are sampled when tc-sample is used and that reported
metadata is correct. Two sets of hosts (with and without LAG) are used,
since metadata extraction in mlxsw is a bit different when LAG is
involved.

 # ./tc_sample.sh
 TEST: tc sample rate (forward)                                      [ OK ]
 TEST: tc sample rate (local receive)                                [ OK ]
 TEST: tc sample maximum rate                                        [ OK ]
 TEST: tc sample group conflict test                                 [ OK ]
 TEST: tc sample iif                                                 [ OK ]
 TEST: tc sample lag iif                                             [ OK ]
 TEST: tc sample oif                                                 [ OK ]
 TEST: tc sample lag oif                                             [ OK ]
 TEST: tc sample out-tc                                              [ OK ]
 TEST: tc sample out-tc-occ                                          [ OK ]
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>