Impact: cleanup

linux/percpu.h includes linux/slab.h, which generates circular inclusion
dependencies when trying to switch kmemtrace to use tracepoints instead
of markers.

This patch allows tracing within slab headers' inline functions.
Signed-off-by: NPekka Enberg <penberg@cs.helsinki.fi>
Cc: Eduard - Gabriel Munteanu <eduard.munteanu@linux360.ro>
Cc: paulmck@linux.vnet.ibm.com
LKML-Reference: <1237898630.25315.83.camel@penberg-laptop>
Signed-off-by: NIngo Molnar <mingo@elte.hu>