f2fs: support async discard

Like most filesystems, f2fs will issue discard command synchronously, so
when user trigger fstrim through ioctl, multiple discard commands will be
issued serially with sync mode, which makes poor performance.

In this patch we try to support async discard, so that all discard
commands can be issued and be waited for endio in batch to improve
performance.

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