x86/UV2: Ack BAU interrupt earlier

This patch moves the ack of the BAU interrupt to the beginning
of  the interrupt handler so that there is less possibility of a
lost interrupt and slower response to a shootdown message.

Signed-off-by: Cliff Wickman <cpw@sgi.com>
Link: http://lkml.kernel.org/r/20120116212146.GE5767@sgi.com


Signed-off-by: Ingo Molnar <mingo@elte.hu>