x86, UV: Calculate BAU destination timeout

Calculate the Broadcast Assist Unit's destination timeout period from the
values in the relevant MMR's.

Store it in each cpu's per-cpu BAU structure so that a destination
timeout can be differentiated from a 'plugged' situation in which all
software ack resources are already allocated and a timeout is pending.
That case returns an immediate destination error.

Signed-off-by: Cliff Wickman <cpw@sgi.com>
Cc: gregkh@suse.de
LKML-Reference: <E1OJvNx-0004Zq-RK@eag09.americas.sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>