kernel - TMPFS - Bug fixing pass - vinitvmio(), umount, readdir

kernel - TMPFS - Bug fixing pass - vinitvmio(), umount, readdir

* Call vinitvmio() with a properly bufcache-aligned number of pages

* Do not call vinitvmio() for symlinks or directories (we don't use
  the buffer cache for those vnode types).

* Adjust tmpfs_dir_attach() and tmpfs_dir_detach() so the detach
  code can be called from umount, and call it from umount to fix
  a panic.

* tmpfs_dir_getdotdotdent() needed to be adjusted to handle the
  root vnode's NULL parent pointer.  Fixed empty root directories
  for the mount.

* Acquire TMPFS_NODE_LOCK() around opreations on dnode->tn_dir.tn_dirhead.