PLEASE TEST: IPI deadlock avoidance patch

[Alan Cox]

Willem Jan Withagen wrote:
> Well my dual AMD64 system seemed to be rather stable since I went with a 
> Tyan board.

This makes sense.  Amd64-based systems do fewer IPIs for TLB shootdown
than i386-based systems.  Specifically, Doug's deadlock, which was in
part in the pipe code, would not have occurred there.  Amd64 systems
avoid many ephemeral mappings (that require IPIs for TLB shootdown)
through the use of the direct virtual-to-physical mapping.

Alan