This fixes two bugs:

The most relavant was that we did not increase buddy->wait_pri if the
new priority was higher, which could lead to all kinds of weird effects,
for example:

- starvation/lock up
- LRU removing all of the cache

because there were requests waiting, but buddy_wait_work() did not see
them.

The second bug is that we also called buddy_wait_work() if there
actually was no change (the critical region could find state ==
IW_SIGNALLED).

Also, for the new assertions, we need to set the proper wait_pri for the
case that the priority is lowered.