Different lagg protocols have different means and policies to process incoming traffic. For example, for `failover ` protocol, by default received traffic is only accepted when they are received through the active port. For `lacp` protocol, LACP control messages are tapped off, also traffic will be dropped if they are received through the port which is in `collecting` state or is not joined to the active aggregator. It confuses if user dump and see inbound traffic on lagg(4) interface but they are actually dropped and not passed into net stack.
Tap traffic after protocol processing so that user will have consistent view of the inbound traffic, meanwhile mbuf is set with correct receiving interface and bpf(4) will diagnose the right direction of inbound packets.
PR: [[https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=270417|270417]]
MFC after: 1 week