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>

arch/x86/platform/uv/tlb_uv.c

@@ -1218,6 +1218,7 @@ void uv_bau_message_interrupt(struct pt_regs *regs)
 	struct ptc_stats *stat;
 	struct msg_desc msgdesc;
 
+	ack_APIC_irq();
 	time_start = get_cycles();
 
 	bcp = &per_cpu(bau_control, smp_processor_id());
@@ -1247,8 +1248,6 @@ void uv_bau_message_interrupt(struct pt_regs *regs)
 		stat->d_nomsg++;
 	else if (count > 1)
 		stat->d_multmsg++;
-
-	ack_APIC_irq();
 }
 
 /*