NULL VFS: store PFS mount points for later use
* This is a hack to obtain the pfs mount point in vop_write()
* PFSes use nullfs mounts. Unfortunately, the nullfs implementation
does not respect the traditional vfs/vnode layers and its vnodes
may belong to many mount point at once.
* What's worse, the mount point passed to VFS functions is the one
of the underlying non-PFS filesystem.
* We use the namecache layer to put the upper mount point of the
PFS in a new vnode field to work around this limitation.