f2fs: fix is_idle() check for discard type

The discard thread should issue upto dpolicy->max_requests at once
and wait for all those discard requests at once it reaches
dpolicy->max_requests. It should then sleep for dpolicy->min_interval
timeout before issuing the next batch of discard requests. But in the
current code of is_idle(), it checks for dcc_info->queued_discard and
aborts issuing the discard batch of max_requests. This
dcc_info->queued_discard will be true always once one discard command
is issued.

It is thus resulting into this type of discard request pattern -

- Issue discard request#1
- is_idle() returns false, discard thread waits for request#1 and then
  sleeps for min_interval 50ms.
- Issue discard request#2
- is_idle() returns false, discard thread waits for request#2 and then
  sleeps for min_interval 50ms.
- and so on for all other discard requests, assuming f2fs is idle w.r.t
  other conditions.

With this fix, the pattern will look like this -

- Issue discard request#1
- Issue discard request#2
  and so on upto max_requests of 8
- Issue discard request#8
- wait for min_interval 50ms.

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