f2fs: reduce cmd_lock coverage in __issue_discard_cmd

__submit_discard_cmd may lead long latency due to exhaustion of I/O
request resource in block layer, so issuing all discard under cmd_lock
may lead to hangtask, in order to avoid that, let's reduce it's coverage.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -1158,14 +1158,14 @@ static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
 	int i, iter = 0, issued = 0;
 	bool io_interrupted = false;
 
-	mutex_lock(&dcc->cmd_lock);
-	f2fs_bug_on(sbi,
-		!__check_rb_tree_consistence(sbi, &dcc->root));
-	blk_start_plug(&plug);
 	for (i = MAX_PLIST_NUM - 1; i >= 0; i--) {
 		if (i + 1 < dpolicy->granularity)
 			break;
 		pend_list = &dcc->pend_list[i];
+
+		mutex_lock(&dcc->cmd_lock);
+		f2fs_bug_on(sbi, !__check_rb_tree_consistence(sbi, &dcc->root));
+		blk_start_plug(&plug);
 		list_for_each_entry_safe(dc, tmp, pend_list, list) {
 			f2fs_bug_on(sbi, dc->state != D_PREP);
 
@@ -1179,12 +1179,14 @@ static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
 			issued++;
 skip:
 			if (++iter >= dpolicy->max_requests)
-				goto out;
+				break;
 		}
+		blk_finish_plug(&plug);
+		mutex_unlock(&dcc->cmd_lock);
+
+		if (iter >= dpolicy->max_requests)
+			break;
 	}
-out:
-	blk_finish_plug(&plug);
-	mutex_unlock(&dcc->cmd_lock);
 
 	if (!issued && io_interrupted)
 		issued = -1;