selftests: tc: Add generic mpls matching support for tc-flower

Add tests in tc_flower.sh for generic matching on MPLS Label Stack
Entries. The label, tc, bos and ttl fields are tested for the first
and second labels. For each field, the minimal and maximal values are
tested (the former at depth 1 and the later at depth 2).
There are also tests for matching the presence of a label stack entry
at a given depth.

In order to reduce the amount of code, all "lse" subcommands are tested
in match_mpls_lse_test(). Action "continue" is used, so that test
packets are evaluated by all filters. Then, we can verify if each
filter matched the expected number of packets.

Some versions of tc-flower produced invalid json output when dumping
MPLS filters with depth > 1. Skip the test if tc isn't recent enough.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>