mm/swap: make NODE_DATA an inline function on CONFIG_FLATMEM

make W=1 generates the following warning in mm/workingset.c for allnoconfig

  mm/workingset.c: In function `unpack_shadow':
  mm/workingset.c:201:15: warning: variable `nid' set but not used [-Wunused-but-set-variable]
    int memcgid, nid;
                 ^~~

On FLATMEM, NODE_DATA returns a global pglist_data without dereferencing
nid.  Make the helper an inline function to suppress the warning, add type
checking and to apply any side-effects in the parameter list.

Link: https://lkml.kernel.org/r/20210520084809.8576-15-mgorman@techsingularity.netSigned-off-by: Mel Gorman <mgorman@techsingularity.net>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mmzone.h

@@ -1064,7 +1064,10 @@ extern char numa_zonelist_order[];
 #ifndef CONFIG_NUMA
 
 extern struct pglist_data contig_page_data;
-#define NODE_DATA(nid)		(&contig_page_data)
+static inline struct pglist_data *NODE_DATA(int nid)
+{
+	return &contig_page_data;
+}
 #define NODE_MEM_MAP(nid)	mem_map
 
 #else /* CONFIG_NUMA */