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.