Check tmpfs_rmdired_p after tmpfs_vnode_get when walking up the tree.

tmpfs_vnode_get drops all locks except possibly the reclaiming bit
lock to keep the tmpfs node from being reclaimed while we're still
interested in it.  Consequently, it does not keep the directory's
existence invariant, so we must check that after tmpfs_vnode_get.

Fixes PR kern/46990.  Tested by Wolfgang Stukenbrock.

(riastradh)