[PATCH] iounmap debugging

We get sporadic reports of `__iounmap: bad address' coming out.  Add a
dump_stack() to find the culprit.

Try to identify which subsystem is having iounmap() problems.

Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>