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.comSigned-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>

include/linux/mm.h

@@ -3122,7 +3122,6 @@ int drop_caches_sysctl_handler(struct ctl_table *, int, void *, size_t *,
 #endif
 
 void drop_slab(void);
-void drop_slab_node(int nid);
 
 #ifndef CONFIG_MMU
 #define randomize_va_space 0

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;