mount: Add ability to restore ro tmpfs

In case if we've dumped read only tmpfs we fail restoring it
because it's mounted with ro flags. Lets mount it with rw,
restore content and then remount as ro.

upd (by xemul@): any fs with restore method likely to
need rw permission on restore.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

mount.c

@@ -1841,20 +1841,22 @@ skip_parent:
 	return 0;
 }
 
-#define MS_CHANGE_TYPE_MASK	\
-	(MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE)
-
 static int do_new_mount(struct mount_info *mi)
 {
+	unsigned long mflags = MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE;
 	char *src;
 	struct fstype *tp = mi->fstype;
+	bool remount_ro = (tp->restore && mi->flags & MS_RDONLY);
 
 	src = resolve_source(mi);
 	if (!src)
 		return -1;
 
+	if (remount_ro)
+		mflags |= MS_RDONLY;
+
 	if (mount(src, mi->mountpoint, tp->name,
-			mi->flags & ~MS_CHANGE_TYPE_MASK, mi->options) < 0) {
+			mi->flags & ~mflags, mi->options) < 0) {
 		pr_perror("Can't mount at %s", mi->mountpoint);
 		return -1;
 	}
@@ -1869,6 +1871,9 @@ static int do_new_mount(struct mount_info *mi)
 	if (tp->restore && tp->restore(mi))
 		return -1;
 
+	if (remount_ro)
+		return mount(NULL, mi->mountpoint, tp->name,
+			     MS_REMOUNT | MS_RDONLY, NULL);
 	return 0;
 }