SLUB: fix locking for hotplug callbacks

Hotplug callbacks are performed with interrupts enabled.  Slub requires
interrupts to be disabled for flushing caches.

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Cc: Michal Piotrowski <michal.k.k.piotrowski@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>