devfs: Fix namecache invalidation for alias nodes

* The devfs alias nodes don't have a directly associated vnode, they
  use the vnode of their target link.

* Unfortunately, the namecache invalidation code in devfs_freep() only
  processed vnodes registered in the devfs_node structure, missing the
  aliases.

* Put the namecache invalidation code in devfs_unlinkp() and make sure
  devfs alias node entries are properly invalidated.