mtrr atomicity fix

Rafael gets this on an SMP box with kernel preemption enabled, during
hibernation and restore (100% of the time):

Enabling non-boot CPUs ...
BUG: using smp_processor_id() in preemptible [00000001] code: bash/4514
caller is mtrr_save_state+0x9/0x40

Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>