mm: Fix signal SIGFPE in slabinfo.c.

In function slab_stats(), if total_free is equal zero, it will error.
So fix it.

Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: majianpeng <majianpeng@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>