losetup: report lost loop devices for finding free loop

losetup: report lost loop devices for finding free loop

commit a6ca0456cc6d("loopdev: report lost loop devices") forgot to
report lost loop device for finding free loop case. With this path,
it will show the following if next free loop device is lost.

 # ./losetup -f
 /dev/loop1 (lost)
 # ./losetup -f /tmp/test.img
 lt-losetup: /tmp/test.img: failed to set up loop device: No such file or directory
 lt-losetup: device node /dev/loop1 (7:1) is lost. You may use mknod(1) to recover it.

Signed-off-by: Junxiao Bi <junxiao.bi@oracle.com>