f2fs: hurry up to issue discard after io interruption

commit e6c6de18f010d9a7d592f4044d2c30213cb3a7bc upstream.

Once we encounter I/O interruption during issuing discards, we will delay
long time before next round, but if system status is I/O idle during the
time, it may loses opportunity to issue discards. So this patch changes
to hurry up to issue discard after io interruption.

Besides, this patch also fixes to issue discards accurately with assigned
rate.

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