f2fs: don't issue flush in f2fs_flush_device_cache() for nobarrier case

This patch changes f2fs_flush_device_cache() to skip issuing flush for
nobarrier case.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -759,6 +759,9 @@ int f2fs_flush_device_cache(struct f2fs_sb_info *sbi)
 	if (!f2fs_is_multi_device(sbi))
 		return 0;
 
+	if (test_opt(sbi, NOBARRIER))
+		return 0;
+
 	for (i = 1; i < sbi->s_ndevs; i++) {
 		if (!f2fs_test_bit(i, (char *)&sbi->dirty_device))
 			continue;