pf: fix potential NULL dereference in SCTP multihome handling

When processing an SCTP ASCONF we re-run the rules processing to check
if the new state should be allowed as well. We used to do so against the
'all' interface, to allow new connections to use any interface.

This is problematic for two reasons, the first being it may unexpectedly
bypass interface restrictions. The more important one is that it
can trigger panics. If the ruleset contains a rule which filters on
interface group we'd attempt to process the group list for the 'all'
interface. As this isn't a real interface it doesn't have an associated
struct ifnet, and we end up dereferencing a NULL pointer.

Solve this by not overriding the interface, instead leaving the physical
interface the SCTP ASCONF arrived on. This implies that we may end up
binding to that interface (if if-bound), and thus denying traffic on
other interfaces. Users can allow this anyway by setting 'state-policy
floating' on the relevant SCTP rules. This arguably better reflects user
intent as well. That is, we'll consider SCTP multihomed states to be
floating if we're in floating mode, and if-bound if we're if-bound.

Update the test cases to account for this, while adding a "pass on
lo" (i.e. pass on an interface group") rule to provoke this issue. Add
separate test cases for the floating and if-bound scenarios.

Reported by: Franco Fichtner <franco@opnsense.org>
MFC after: 3 weeks
Sponsored by: Orange Business Services