VFS accounting: check if nullfs mp* really exist

VFS accounting: check if nullfs mp* really exist

* Nullfs are a clever hack, they forget their mp after mount and let the
  non-nullfs underlying mount point do all the work

* Nevertheless, we need to get the real struct mount associated to a mount
  point to count the space used by PFSes. We cache it in the vnode.

* But since the vnode we use really belongs to the lowest underlying
  mount point, the upper-level mp may already have been freed.

* We need to first check if it still valid; a new function,
  mountlist_exists() now exists for that purpose.

* Should fix issue #2266