losetup: use loop-control to explicitly ask for device

losetup: use loop-control to explicitly ask for device

Now we use LOOP_CTL_GET_FREE ioctl to ask for free device, for example

  losetup -f foo.img

Unfortunately, losetup(8) allows to ask for specified device

  losetup /dev/loop100 foo.img

and in this case we assume that the device already exists in the
system. This is incorrect, we should be able to use loop-control
LOOP_CTL_ADD ioctl to ask for the specified device.

Signed-off-by: Karel Zak <kzak@redhat.com>