pf: skip urpf check for sctp multihomed states

When we create a new state for multihomed sctp connections (i.e.
based on INIT/INIT_ACK or ASCONF parameters) we cannot know what
interfaces we'll be seeing that traffic on. These states are floating
states, i.e. on "all" interfaces. We cannot do reverse path filtering
for these states, so do not do so.

MFC after: 1 week
Sponsored by: Orange Business Services