Do not return /dev/loop-control in unused_loop_device

The function unused_loop_device in /usr/libexec/zfs/common.sh
returns /dev/loop-control on the first call. This device is NOT
a loop device (https://github.com/torvalds/linux/commit/770fe30)
it is a control device. This in turn causes the script zconfig.sh
to fail with:

zpool-create.sh: Error 1 creating /tmp/zpool-vdev0 ->
/dev/loop-control loopback

The patch makes the function return /dev/loop[0-9]* which are
loop devices.

Signed-off-by: Andrew Reid <ColdCanuck@nailedtotheperch.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #797