f2fs: check discard command number before traversing discard pending list

In trim thread, let's add a condition to check discard command number
before traversing discard pending list, it can avoid unneeded traversing
if there is no discard command.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -1759,6 +1759,8 @@ static int issue_discard_thread(void *data)
 			wait_ms = dpolicy.max_interval;
 			continue;
 		}
+		if (!atomic_read(&dcc->discard_cmd_cnt))
+			continue;
 
 		if (sbi->gc_mode == GC_URGENT_HIGH)
 			__init_discard_policy(sbi, &dpolicy, DPOLICY_FORCE, 1);