f2fs: relax async discard commands more

This patch relaxes async discard commands to avoid waiting its end_io during
checkpoint.
Instead of waiting them during checkpoint, it will be done when actually reusing
them.

Test on initial partition of nvme drive.

 # time fstrim /mnt/test

Before : 6.158s
After : 4.822s

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