NULL VFS: store PFS mount points for later use

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.