MFC r272349, r272422 and r272479:
- Fix XHCI driver for devices which have more than 15 physical root HUB
  ports. The current bitmap array was too small to hold more than 16
  bits and would at some point toggle the context size, which then would
  trigger an enumeration fault and cause a fallback to the EHCI
  companion controller, if any.
- Make sure we always set the maximum number of valid contexts.
- Set default cycle state in case of early interrupts.
MFC r272480:
When we fail to get a USB reference we should just return, because
there are no more references held.
MFC 272550:

  Remove stray uma_mtx lock/unlock in zone_drain_wait()

  Callers of zone_drain_wait(M_WAITOK) do not need to hold (and were not)
  the uma_mtx, but we would attempt to unlock and relock the mutex if we
  had to sleep because the zone was already draining. The M_NOWAIT callers
  may hold the uma_mtx, but we do not sleep in that case.
MFC r272762: Correct scale factor for T terabyte suffix

PR:		194250