mnt: don't match the wrong nested bind mount

Andrey reported this issue and it took me a while to figure out exactly what
might cause it. I think the comment describes it accurately, as with that
example I end up with mountinfo on the host like:

47 23 253:1 /root/bind1/subdir /root/bind2 rw,relatime shared:1 - ext4 /dev/disk/by-uuid/6c5a78e0-95fa-49a8-aa91-a8093d295e58 rw,data=ordered
48 23 253:1 /root/bind1 /root/bind3 rw,relatime shared:1 - ext4 /dev/disk/by-uuid/6c5a78e0-95fa-49a8-aa91-a8093d295e58 rw,data=ordered

Reported-by: Andrew Vagin <avagin@odin.com>
CC: Andrew Vagin <avagin@odin.com>
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Acked-by: Andrew Vagin <avagin@odin.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>