cache_fullpath - Guess mountpoints if requested

cache_fullpath - Guess mountpoints if requested

* cache_fullpath (and vn_fullpath) now take an extra parameter, guess,
  which, if != 0, makes cache_fullpath look for a matching mp if an ncp
  flagged as a mountpoint is found while traversing upwards.
  This fixes uses of *_fullpath when no nch is provided, but only a vp.

* Change all consumers of cache_fullpath and vn_fullpath to accomodate
  for the extra parameter.

Suggested-by: Matthew Dillon