f2fs: avoid race in between atomic_read & atomic_inc

Previously, we will miss merging flush command during fsync due to below
race condition:

Thread A 		Thread B		Thread C
- f2fs_issue_flush
 - atomic_read(&issing_flush)
			- f2fs_issue_flush
			 - atomic_read(&issing_flush)
						- f2fs_issue_flush
						 - atomic_read(&issing_flush)
  - atomic_inc(&issing_flush)
			  - atomic_inc(&issing_flush)
						  - atomic_inc(&issing_flush)
   - submit_flush_wait
			   - submit_flush_wait
						   - submit_flush_wait

It needs to use atomic_inc_return instead to avoid such race.
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -536,8 +536,7 @@ int f2fs_issue_flush(struct f2fs_sb_info *sbi)
 		return ret;
 	}
 
-	if (!atomic_read(&fcc->issing_flush)) {
-		atomic_inc(&fcc->issing_flush);
+	if (atomic_inc_return(&fcc->issing_flush) == 1) {
 		ret = submit_flush_wait(sbi);
 		atomic_dec(&fcc->issing_flush);
 
@@ -547,7 +546,6 @@ int f2fs_issue_flush(struct f2fs_sb_info *sbi)
 
 	init_completion(&cmd.wait);
 
-	atomic_inc(&fcc->issing_flush);
 	llist_add(&cmd.llnode, &fcc->issue_list);
 
 	/* update issue_list before we wake up issue_flush thread */