exfat: call sync_filesystem for read-only remount

We need to commit dirty metadata and pages to disk
before remounting exfat as read-only.

This fixes a failure in xfstests generic/452

generic/452 does the following:
cp something <exfat>/
mount -o remount,ro <exfat>

the <exfat>/something is corrupted. because while
exfat is remounted as read-only, exfat doesn't
have a chance to commit metadata and
vfs invalidates page caches in a block device.
Signed-off-by: Hyunchul Lee <hyc.lee@gmail.com>
Acked-by: Sungjong Seo <sj1557.seo@samsung.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>

fs/exfat/super.c

@@ -693,10 +693,20 @@ static void exfat_free(struct fs_context *fc)
 	}
 }
 
+static int exfat_reconfigure(struct fs_context *fc)
+{
+	fc->sb_flags |= SB_NODIRATIME;
+
+	/* volume flag will be updated in exfat_sync_fs */
+	sync_filesystem(fc->root->d_sb);
+	return 0;
+}
+
 static const struct fs_context_operations exfat_context_ops = {
 	.parse_param	= exfat_parse_param,
 	.get_tree	= exfat_get_tree,
 	.free		= exfat_free,
+	.reconfigure	= exfat_reconfigure,
 };
 
 static int exfat_init_fs_context(struct fs_context *fc)