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