f2fs: add app/fs io stat

This patch enables inner app/fs io stats and introduces below virtual fs
nodes for exposing stats info:
/sys/fs/f2fs/<dev>/iostat_enable
/proc/fs/f2fs/<dev>/iostat_info

Signed-off-by: Chao Yu <yuchao0@huawei.com>
[Jaegeuk Kim: fix wrong stat assignment]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 5c1ac2c..ec9fd73 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -2060,6 +2060,10 @@
 	set_sbi_flag(sbi, SBI_POR_DOING);
 	spin_lock_init(&sbi->stat_lock);
 
+	/* init iostat info */
+	spin_lock_init(&sbi->iostat_lock);
+	sbi->iostat_enable = false;
+
 	for (i = 0; i < NR_PAGE_TYPE; i++) {
 		int n = (i == META) ? 1: NR_TEMP_TYPE;
 		int j;