lagg(4): Tap traffic after protocol processing

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 not in collecting state or is not joined to
the active aggregator. It confuses if user dump and see inbound traffic on
lagg(4) interfaces but they are actually silently dropped and not passed into
the 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: 270417
Reviewed by: melifaro (previous version)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D39225