iproute2: fix MPLS label parsing

iproute2: fix MPLS label parsing

The initial value of "label" in parse_mpls() is 0xffffffff. Therefore
we should test for this value, and not 0, to detect if a label has been
provided. The "!label" test not only fails to detect a missing label
parameter, it also prevents the use of the IPv4 explicit NULL label,
which actually equals 0.

Reproducer:
  $ ip link add name dm0 type dummy
  $ tc qdisc add dev dm0 ingress

  $ tc filter add dev dm0 parent ffff: matchall action mpls push
  Error: act_mpls: Label is required for MPLS push.
  We have an error talking to the kernel
  --> Filter was pushed to the kernel, where it got rejected.

  $ tc filter add dev dm0 parent ffff: matchall action mpls push label 0
  Error: argument "label" is required
  --> Label 0 was rejected by iproute2.

Expected result:
  $ tc filter add dev dm0 parent ffff: matchall action mpls push
  Error: argument "label" is required
  --> Filter was directly rejected by iproute2.

  $ tc filter add dev dm0 parent ffff: matchall action mpls push label 0
  --> Filter is accepted.

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