kernel - Fix fstatfs() and fstatvfs() when called from a null-mounted chroot()

kernel - Fix fstatfs() and fstatvfs() when called from a null-mounted chroot()

* These funtions were using the wrong starting mount pointer when calling
  cache_fullpath().  They were using the mount pointer for the open file's
  vnode instead of the overlay (null) mount pointer.

  This caused cache_fullpath() to believe the fd represented a file outside
  the chroot and return an error.

Reported-by: Francois Tigeot<ftigeot@wolfpond.org>: