addr: Add support for AF_MPLS
addr: Add support for AF_MPLS

MPLS 'addresses' are 4-byte labels with a prefix length of 20.

Signed-off-by: David Ahern <b80c1600f604d3b0d768f26f90a76757e76005dd@gmail.com>

tklauser/libnl,tklauser/libnl,tklauser/libnl,greearb/libnl-ct,thom311/libnl,thom311/libnl,greearb/libnl-ct,tklauser/libnl,greearb/libnl-ct,thom311/libnl