vmscan: make drop_slab_node static

drop_slab_node is only used in drop_slab.  So remove it's declaration
from header file and add keyword static for it's definition.

Link: https://lkml.kernel.org/r/20211111062445.5236-1-ligang.bdlg@bytedance.com
Signed-off-by: Gang Li <ligang.bdlg@bytedance.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 700434d..090bfb6 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -951,7 +951,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
 	return freed;
 }
 
-void drop_slab_node(int nid)
+static void drop_slab_node(int nid)
 {
 	unsigned long freed;
 	int shift = 0;