make mpls_input take a struct ifnet *ifp argument.

make mpls_input take a struct ifnet *ifp argument.

this makes it like all our other protocol family input functions.

mpls_input always looks up the interface the mbuf was received on,
but it's always called by code that already has a reference to that
interface anyway. the result of this is a few less if_get/if_put
calls.

ok mpi@ bluhm@ visa@ claudio@