lvmstrap: add an explicit test for mounted devices

Recent kernels/userland report a mounted filesystem as follows:

    root@node4:~# fuser -avm /dev/sda5
                         USER        PID ACCESS COMMAND
    /dev/sda5:           root     kernel mount /srv/ganeti

But old ones do not:

    node1# fuser -avm /dev/sda5
                         USER        PID ACCESS COMMAND
    /dev/sda5:

So in order to cover both, we add an explicit test for this. We
already compute the device numbers of all mounted filesystems, so it
is a trivial addition (GetMountInfo() is cheap, so we don't need to
cache it).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>