another mb() for smpboot.c

From: Benjamin LaHaise <bcrl@kvack.org>

The following is probably a good idea given that the atomic_set() isn't 
a barrier here either.

Signed-off-by: Benjamin LaHaise <benjamin.c.lahaise@intel.com>
Signed-off-by: Andi Kleen <ak@suse.de>

diff --git a/arch/x86_64/kernel/smpboot.c b/arch/x86_64/kernel/smpboot.c
index 10b951b..a9a8f97 100644
Index: linux/arch/x86_64/kernel/smpboot.c
===================================================================
--- linux.orig/arch/x86_64/kernel/smpboot.c
+++ linux/arch/x86_64/kernel/smpboot.c
@@ -646,6 +646,7 @@ static int __cpuinit wakeup_secondary_vi
 		send_status = apic_read(APIC_ICR) & APIC_ICR_BUSY;
 	} while (send_status && (timeout++ < 1000));
 
+	mb();
 	atomic_set(&init_deasserted, 1);
 
 	num_starts = 2;