f2fs: introduce max_ordered_discard sysfs node

The current max_ordered_discard is a fixed value, change it to be
configurable through the sys node.

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