Allow prio_tree_next to be used as the only function for tree traversal,
similar to how vma_prio_tree_next works.

This patch isn't needed for the generalization, but since it affects the API,
it's better to include it first.
Signed-off-by: Werner Almesberger <werner@almesberger.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>