f2fs: enable real-time discard by default

f2fs is focused on flash based storage, so let's enable real-time
discard by default, if user don't want to enable it, 'nodiscard'
mount option should be used on mount.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>