slub: tid must be retrieved from the percpu area of the current processor

As Steven Rostedt has pointer out: rescheduling could occur on a
different processor after the determination of the per cpu pointer and
before the tid is retrieved. This could result in allocation from the
wrong node in slab_alloc().

The effect is much more severe in slab_free() where we could free to the
freelist of the wrong page.

The window for something like that occurring is pretty small but it is
possible.

Change-Id: Ie558aaf74d2219bae0f3b4b5fe87d1346f10ea55
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>