mm/sparse: optimize sparse_index_alloc

With CONFIG_SPARSEMEM_EXTREME, the two levels of memory section
descriptors are allocated from slab or bootmem.  When allocating from
slab, let slab/bootmem allocator clear the memory chunk.  We needn't clear
it explicitly.
Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/sparse.c

@@ -65,14 +65,12 @@ static struct mem_section noinline __init_refok *sparse_index_alloc(int nid)
 
 	if (slab_is_available()) {
 		if (node_state(nid, N_HIGH_MEMORY))
-			section = kmalloc_node(array_size, GFP_KERNEL, nid);
+			section = kzalloc_node(array_size, GFP_KERNEL, nid);
 		else
-			section = kmalloc(array_size, GFP_KERNEL);
-	} else
+			section = kzalloc(array_size, GFP_KERNEL);
+	} else {
 		section = alloc_bootmem_node(NODE_DATA(nid), array_size);
-
-	if (section)
-		memset(section, 0, array_size);
+	}
 
 	return section;
 }