Here is the patch that matches the changes I made for RHAS w.r.t
perfmon and the O(1) scheduler. This fixes the deadlocks we were
seing in RHAS and therefore on 2.5 as well.

The key change is that SIGPROF notifications are now delivered
in the perfmon kernel exit handler (called from entry.S).
At that point, we know none of the runqueue locks are held.

I have run the same test suite I used on RHAS overnight and no
sign of deadlocks.