kernel - Add overlapping buffer test to the buffer cache

kernel - Add overlapping buffer test to the buffer cache

* Add a test for overlapping buffer in bgetvp().

* Add a sysctl vfs.check_buf_overlap which defaults to enabling the
  check invasively.  Overlapping buffer cache buffers can be deadly.

  0 - disabled
  1 - checked and reported
  2 - checked and panic (invasive) (default)