loopback: Do not automaticaly replace existing loopback dev, error instead

loopback: Do not automaticaly replace existing loopback dev, error instead

If there is a loopback device with the same name as the one to be created,
instead of closing the old one and replacing it with the new one, return an
error instead. If the loopback device was created, its probably being used
by something and just replacing it may cause GRUB to crash unexpectedly.
This fixes obvious problems like "loopback d (d)/somefile". Its not too
onerous to force the user to delete the loopback first with the "-d" switch.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>