f2fs: export compress_percent and compress_watermark entries

This patch export below sysfs entries for better control cached
compress page count.

/sys/fs/f2fs/<disk>/compress_watermark
/sys/fs/f2fs/<disk>/compress_percent

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