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>