slub: use count_partial_free_approx() in slab_out_of_memory()

slab_out_of_memory() uses count_partial() to get the exact count
of free objects for each node. As it may get called in the slab
allocation path, count_partial_free_approx() can be used to avoid
the risk and overhead of traversing a long partial slab list.

At the same time, show_slab_objects() still uses count_partial().
Thus, slub users can still have the option to access the exact
count of objects via sysfs if the overhead is acceptable to them.

Signed-off-by: Jianfeng Wang <jianfeng.w.wang@oracle.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

mm/slub.c

@@ -3301,7 +3301,7 @@ slab_out_of_memory(struct kmem_cache *s, gfp_t gfpflags, int nid)
 		unsigned long nr_objs;
 		unsigned long nr_free;
 
-		nr_free  = count_partial(n, count_free);
+		nr_free  = count_partial_free_approx(n);
 		nr_slabs = node_nr_slabs(n);
 		nr_objs  = node_nr_objs(n);