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>
3 _is_ a magic number, while 0-7 is a common number seen, this failure on the devicemapper unit-test will only needs as many free loops as it has tests. This number is 4. So 7 is OK, I reckon.