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.