kernel - Enhance getcacheblk() (improve saturated write performance (3)).

kernel - Enhance getcacheblk() (improve saturated write performance (3)).

* Change getcacheblk() to use getblk() instead of findblk() when the
  block size is known, allowing a cached buffer to be reinstantiated
  from VM backing store in the MPSAFE path.

  Well, mostly MPSAFE, the vm_token is still acquired, but this is still
  much faster than what HAMMER does when the buffer is not in-cache.

* This more than doubles blogbench performance w/HAMMER and further reduces
  concurrent read vs write conflicts.