kernel - Cut buffer cache related pmap invalidations in half

kernel - Cut buffer cache related pmap invalidations in half

* Do not bother to invalidate the TLB when tearing down a buffer
  cache buffer.  On the flip side, always invalidate the TLB
  (the page range in question) when entering pages into a buffer
  cache buffer.  Only applicable to normal VMIO buffers.

* Significantly improves buffer cache / filesystem performance with
  no real risk.

* Significantly improves performance for tmpfs teardowns on unmount
  (which typically have to tear-down a lot of buffer cache buffers).