slabinfo: fall back from /sys/kernel/slab to /sys/slab

I keep running upstream and mm kernels and the location of the slab
directory is different since upstream still uses /sys/slab.  This patch
makes slabinfo check /sys/slab if /sys/kernel/slab is not there.  Makes
slabinfo work on any kernel.

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>