f2fs: support accounting iostat count and avg_bytes

Previously, we supported to account iostat io_bytes,
in this patch, it adds to account iostat count and avg_bytes:

time:           1671648667
                        io_bytes         count            avg_bytes
[WRITE]
app buffered data:      31               2                15

Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>