If something else on the system is using loopback devices, then the
device that's free at the call to `losetup -f` may not be free in the
following call to try to use it. Instead, find and use the first free
loopback device in a single call to losetup.
Signed-off-by: Will Thompson <wjt@endlessm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
