Export prio_tree functions such that they can be used by other subsystems than
only VMAs.  Also adds a mode to prio_tree to use it with keys explicitly
included in the prio_tree meta-data.

The plan is to also consider converting VMAs to use explicit keys, so that the
old "raw" mode can be removed.
Signed-off-by: Werner Almesberger <werner@almesberger.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>