Use this combination to mean that if the calling thread runs into a page
shortage and has to wait in VM_WAIT, then it can be waken up and allowed
to try to grab memory when available pages go above v_pageout_free_min
as opposed to much higher (by default) v_free_min threshold.
Thus, M_USE_RESERVE | M_WAITOK sets the same threshold as automatically
given to the page daemon.
Use M_USE_RESERVE | M_WAITOK for illumos KM_PUSHPAGE, which used in
ZFS ARC code, which could be critical to pageout and ARC size reduction.