f2fs: turn down IO priority of discard from background

In order to avoid interfering normal r/w IO, let's turn down IO
priority of discard issued from background.
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -931,6 +931,7 @@ static void __init_discard_policy(struct f2fs_sb_info *sbi,
 		dpolicy->min_interval = DEF_MIN_DISCARD_ISSUE_TIME;
 		dpolicy->max_interval = DEF_MAX_DISCARD_ISSUE_TIME;
 		dpolicy->io_aware = true;
+		dpolicy->sync = false;
 		if (utilization(sbi) > DEF_DISCARD_URGENT_UTIL) {
 			dpolicy->granularity = 1;
 			dpolicy->max_interval = DEF_MIN_DISCARD_ISSUE_TIME;