tc: flower: support multiple MPLS LSE match

tc: flower: support multiple MPLS LSE match

Add the new "mpls" keyword that can be used to match MPLS fields in
arbitrary Label Stack Entries.
LSEs are introduced by the "lse" keyword and followed by LSE options:
"depth", "label", "tc", "bos" and "ttl". The depth is manadtory, the
other options are optionals.

For example, the following filter drops MPLS packets having two labels,
where the first label is 21 and has TTL 64 and the second label is 22:

$ tc filter add dev ethX ingress proto mpls_uc flower mpls \
    lse depth 1 label 21 ttl 64 \
    lse depth 2 label 22 bos 1 \
    action drop

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>