MFC r240938, r240942: Avoid INVARIANTS panic destroying an in-use tap(4)

  The requirement (implied by the KASSERT in tap_destroy) that the tap is
  closed isn't valid; destroy_dev will block in devdrn while other threads
  are in d_* functions.

  Note: if_tun had the same issue, addressed in SVN revisions r186391,
  r186483 and r186497.  The use of the condvar there appears to be
  redundant with the functionality provided by destroy_dev.

PR:		172075
Approved by:	re