usbnet: Defer hardware multicast filter updates to USB task.

Breaks deadlock:

- usbnet_detach holds usbnet lock, awaits kpause in ure_reset
- callout holds softclock `lock' (sequential softints, blocks kpause
  wakeup), awaits softnet_lock in tcp_timer_keep, frag6_fasttimo, &c.
- soclose holds softnet_lock, awaits usbnet lock in SIOCDELMULTI

This change breaks the deadlock by not passing the SIOCADDMULTI or
SIOCDELMULTI ioctl synchronously to the driver, which typically takes
the usbnet lock.

With this change, the ethernet layer still maintains the list of
multicast addresses synchronously, but we defer the driver logic that
updates the hardware multicast filter to an asynchronous USB task
without softnet_lock held.

This doesn't cause exactly the same ioctl to be sent to the driver --
usbnet just sends SIOCDELMULTI with an all-zero struct ifreq, and
might drop some ioctls if issued in quick succession.  This is OK
because none of the drivers actually distinguish between SIOCADDMULTI
and SIOCDELMULTI, or examine the argument; the drivers just commit
whatever multicast addresses are listed in the ethercom.

Other than the different ioctl submitted, there is no change to the
ABI or locking scheme of usbnet, so this is safe to pull up to
netbsd-9.  This means we unfortunately can't guarantee that if a
process issues SIOCADDMULTI and then sendto, the multicast filter
update will be done by the time of the sendto -- and, more
importantly, the packets received in reply to it.  But failing to
guarantee that is better than deadlocking!  Later changes on HEAD
will restore the synchronous multicast filter updates with much more
extensive ABI changes and API simplifications in usbnet(9).

Proposed on tech-net:
https://mail-index.netbsd.org/tech-net/2021/12/30/msg008164.html

XXX pullup-9

(riastradh)