ceph: removed duplicated #includes

fs/ceph/auth.c: linux/slab.h is included more than once.
fs/ceph/super.h: linux/slab.h is included more than once.

Acked-by: Christoph Lameter <cl@linux-foundation.org>
Signed-off-by: Andrea Gelmini <andrea.gelmini@gelma.net>
Signed-off-by: Sage Weil <sage@newdream.net>