f2fs: add app/fs io stat

commit b0af6d491a6b5f5622fa91ac75f34f3640f862c4 upstream.

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>
Git-commit: 0056fb13
Git-repo: https://android.googlesource.com/kernel/common.git


Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>