Mknod more loopbacks for devmapper

Fixes #7058

The change creates additional loopback files for the devicemapper unit tests.  Some systems do not initially create very many so this ensures that we have alteast 7 loop devices available. 

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>
everything that i have seen usually has a beginning count of 7 loopX thingys