f2fs: add nobarrier mount option

This patch adds a mount option, nobarrier, in f2fs.
The assumption in here is that file system keeps the IO ordering, but
doesn't care about cache flushes inside the storages.

Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 34649aa..93593ce 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -52,6 +52,7 @@
 	Opt_inline_xattr,
 	Opt_inline_data,
 	Opt_flush_merge,
+	Opt_nobarrier,
 	Opt_err,
 };
 
@@ -69,6 +70,7 @@
 	{Opt_inline_xattr, "inline_xattr"},
 	{Opt_inline_data, "inline_data"},
 	{Opt_flush_merge, "flush_merge"},
+	{Opt_nobarrier, "nobarrier"},
 	{Opt_err, NULL},
 };
 
@@ -339,6 +341,9 @@
 		case Opt_flush_merge:
 			set_opt(sbi, FLUSH_MERGE);
 			break;
+		case Opt_nobarrier:
+			set_opt(sbi, NOBARRIER);
+			break;
 		default:
 			f2fs_msg(sb, KERN_ERR,
 				"Unrecognized mount option \"%s\" or missing value",
@@ -544,6 +549,8 @@
 		seq_puts(seq, ",inline_data");
 	if (!f2fs_readonly(sbi->sb) && test_opt(sbi, FLUSH_MERGE))
 		seq_puts(seq, ",flush_merge");
+	if (test_opt(sbi, NOBARRIER))
+		seq_puts(seq, ",nobarrier");
 	seq_printf(seq, ",active_logs=%u", sbi->active_logs);
 
 	return 0;