f2fs: make fault injection covering __submit_flush_wait()

This patch changes to allow failure of f2fs_bio_alloc() in
__submit_flush_wait(), which can simulate flush error in checkpoint()
for covering more error paths.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -542,9 +542,13 @@ void f2fs_balance_fs_bg(struct f2fs_sb_info *sbi)
 static int __submit_flush_wait(struct f2fs_sb_info *sbi,
 				struct block_device *bdev)
 {
-	struct bio *bio = f2fs_bio_alloc(sbi, 0, true);
+	struct bio *bio;
 	int ret;
 
+	bio = f2fs_bio_alloc(sbi, 0, false);
+	if (!bio)
+		return -ENOMEM;
+
 	bio->bi_opf = REQ_OP_WRITE | REQ_SYNC | REQ_PREFLUSH;
 	bio_set_dev(bio, bdev);
 	ret = submit_bio_wait(bio);