f2fs: shrink blk plug region

Don't use blk plug covering area where there won't be any IOs being issued.
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -853,9 +853,8 @@ static int issue_discard_thread(void *data)
 	if (kthread_should_stop())
 		return 0;
 
-	blk_start_plug(&plug);
-
 	mutex_lock(&dcc->cmd_lock);
+	blk_start_plug(&plug);
 	list_for_each_entry_safe(dc, tmp, pend_list, list) {
 		f2fs_bug_on(sbi, dc->state != D_PREP);
 
@@ -865,6 +864,7 @@ static int issue_discard_thread(void *data)
 		if (iter++ > DISCARD_ISSUE_RATE)
 			break;
 	}
+	blk_finish_plug(&plug);
 
 	list_for_each_entry_safe(dc, tmp, wait_list, list) {
 		if (dc->state == D_DONE) {
@@ -874,8 +874,6 @@ static int issue_discard_thread(void *data)
 	}
 	mutex_unlock(&dcc->cmd_lock);
 
-	blk_finish_plug(&plug);
-
 	iter = 0;
 	congestion_wait(BLK_RW_SYNC, HZ/50);