mount: fix comparison of stat.st_dev and mount_info->s_dev (v2)

st_dev and s_dev have different formats.
st_dev is (MAJOR(dev) << 8) | MINOR(dev)
s_dev is (MAJOR(dev) << 20) | MINOR(dev)

so we need to convert one of them

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