mm/slab.c: add a helper function get_first_slab

Add a new helper function get_first_slab() that get the first slab from
a kmem_cache_node.

Signed-off-by: Geliang Tang <geliangtang@163.com>
Acked-by: Christoph Lameter <cl@linux.com>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>