mount: handle old images where sb and mnt options were not separated (v3)

Currently we have only one common flag. It is MS_READONLY and it's
interpreted as a super block flag to not change old behaviour.

v2: add a comment
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>