slab, cleanup: remove unneeded return

The procedure ends right after the if-statement, so remove ``return''.
Also move the last common statement outside.

Signed-off-by: Zhao Jin <cronozhj@gmail.com>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>