x86: numa_32.c: fix sparse warning: Using plain integer as NULL pointer

Fix this sparse warning:

  arch/x86/mm/numa_32.c:197:24: warning: Using plain integer as NULL pointer
Signed-off-by: NHannes Eder <hannes@hanneseder.net>
Cc: trivial@kernel.org
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/mm/numa_32.c

@@ -194,7 +194,7 @@ void *alloc_remap(int nid, unsigned long size)
 	size = ALIGN(size, L1_CACHE_BYTES);
 
 	if (!allocation || (allocation + size) >= node_remap_end_vaddr[nid])
-		return 0;
+		return NULL;
 
 	node_remap_alloc_vaddr[nid] += size;
 	memset(allocation, 0, size);