nommu: fix build breakage

Commit 34e55232e59f7b19050267a05ff1226e5cd122a5 ("mm: avoid false sharing
of mm_counter") added sync_mm_rss() for syncing loosely accounted rss
counters.  It's for CONFIG_MMU but sync_mm_rss is called even in NOMMU
enviroment (kerne/exit.c, fs/exec.c).  Above commit doesn't handle it
well.

This patch changes
  SPLIT_RSS_COUNTING depends on SPLIT_PTLOCKS && CONFIG_MMU

And for avoid unnecessary function calls, sync_mm_rss changed to be inlined
noop function in header file.

Reported-by: David Howells <dhowells@redhat.com>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Greg Ungerer <gerg@snapgear.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mm.h

@@ -971,7 +971,13 @@ static inline void setmax_mm_hiwater_rss(unsigned long *maxrss,
 		*maxrss = hiwater_rss;
 }
 
+#if defined(SPLIT_RSS_COUNTING)
 void sync_mm_rss(struct task_struct *task, struct mm_struct *mm);
+#else
+static inline void sync_mm_rss(struct task_struct *task, struct mm_struct *mm)
+{
+}
+#endif
 
 /*
  * A callback you can register to apply pressure to ageable caches.

include/linux/mm_types.h

@@ -203,7 +203,7 @@ enum {
 	NR_MM_COUNTERS
 };
 
-#if USE_SPLIT_PTLOCKS
+#if USE_SPLIT_PTLOCKS && defined(CONFIG_MMU)
 #define SPLIT_RSS_COUNTING
 struct mm_rss_stat {
 	atomic_long_t count[NR_MM_COUNTERS];

mm/memory.c

@@ -190,9 +190,6 @@ static void check_sync_rss_stat(struct task_struct *task)
 {
 }
 
-void sync_mm_rss(struct task_struct *task, struct mm_struct *mm)
-{
-}
 #endif
 
 /*