f2fs, lockdep: annotate mutex_lock_all()

Majianpeng reported a lockdep splat for f2fs. It turns out mutex_lock_all()
acquires an array of locks (in global/local lock style).

Any such operation is always serialized using cp_mutex, therefore there is no
fs_lock[] lock-order issue; tell lockdep about this using the
mutex_lock_nest_lock() primitive.
Reported-by: majianpeng <majianpeng@gmail.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/f2fs.h

@@ -495,9 +495,17 @@ static inline void clear_ckpt_flags(struct f2fs_checkpoint *cp, unsigned int f)
 
 static inline void mutex_lock_all(struct f2fs_sb_info *sbi)
 {
-	int i = 0;
-	for (; i < NR_GLOBAL_LOCKS; i++)
-		mutex_lock(&sbi->fs_lock[i]);
+	int i;
+
+	for (i = 0; i < NR_GLOBAL_LOCKS; i++) {
+		/*
+		 * This is the only time we take multiple fs_lock[]
+		 * instances; the order is immaterial since we
+		 * always hold cp_mutex, which serializes multiple
+		 * such operations.
+		 */
+		mutex_lock_nest_lock(&sbi->fs_lock[i], &sbi->cp_mutex);
+	}
 }
 
 static inline void mutex_unlock_all(struct f2fs_sb_info *sbi)