Allow vlan interfaces to rx through netmap(4).

Normally after receiving a packet, a vlan(4) interface sends the packet
back through its parent interface's rx routine so that it can be
processed as an untagged frame. It does this by using the parent's
ifp->if_input. This is incompatible with netmap(4), which replaces the
vlan(4) interface's if_input with a netmap(4) hook. Fix this by using
the vlan(4) interface's ifp instead of the parent's directly.

Reported by: Harry Schmalzbauer <freebsd@omnilan.de>
Reviewed by: rstone
Approved by: rstone (mentor)
MFC after: 3 days
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D12191