iflib: Stop interface before (un)registering VLAN

This patch is intended to solve a specific problem that iavf(4)
encounters, but what it does can be extended to solve other issues.

To summarize the iavf(4) issue, if the PF driver configures VLAN
anti-spoof, then the VF driver needs to make sure no untagged traffic is
sent if a VLAN is configured, and vice-versa. This can be an issue when
a VLAN is being registered or unregistered, e.g. when a packet may be on
the ring with a VLAN in it, but the VLANs are being unregistered. This
can cause that tagged packet to go out and cause an MDD event.

To fix this, include a new interface-dependent function that drivers can
implement named IFDI_NEEDS_RESTART(). Right now, this function is called
in iflib_vlan_unregister/register() to determine whether the interface
needs to be stopped and started when a VLAN is registered or
unregistered. The default return value of IFDI_NEEDS_RESTART() is true,
so this fixes the MDD problem that iavf(4) encounters, since the
interface rings are flushed during a stop/init.

A future change to iavf(4) will implement that function just in case the
default value changes, and to make it explicit that this interface reset
is required when a VLAN is added or removed.

Reviewed by: gallatin@
MFC after: 1 week
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D22086