x86/mce: Remove explicit smp_rmb() when starting CPUs sync

mce_start() has an explicit smp_wmb() to serialize writes to global_nwo
and mce_callin. However, atomic_inc_return() implies barriers on both
sides of the call, as such simply rely on this full SMP barrier.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Cc: linux-edac <linux-edac@vger.kernel.org>
Link: http://lkml.kernel.org/r/1458602396-840-1-git-send-email-dave@stgolabs.net
Link: http://lkml.kernel.org/r/1459929916-12852-3-git-send-email-bp@alien8.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>