kernel - Improve physio performance

kernel - Improve physio performance

* See http://apollo.backplane.com/DFlyMisc/nvme_sys03.txt

* Hash the pbuf system.  This chops down spin-lock collisions
  at high transaction rates (>150K IOPS) by 1000x.

* Implement a pbuf with pre-allocated kernel memory that we
  copy into, avoiding page table manipulations and thus
  avoiding system-wide invltlb/invlpg IPIs.

* This increases NVMe IOPS tests with three cards from
  150K-200K IOPS to 950K IOPS using physio (random read,
  4K blocks, from urandom-filled partition, with many
  process threads, from 3 NVMe cards in parallel).

* Further adjustments to the vkernel build.