check_free_disk_space: Btrfs has no fixed inode limit

This fixes division by 0.
13 jobs for fix/apachedex-config-file in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Master.Python2

00:24:43

passed SlapOS.Eggs.UnitTest-Master.Python3

00:24:39

passed SlapOS.Eggs.UnitTest-Master.Python2

00:34:40

passed SlapOS.Eggs.UnitTest-Master.Python2

00:13:09

passed SlapOS.Eggs.UnitTest-Master.Python2

00:35:02

passed SlapOS.Eggs.UnitTest-Master.Python2

00:08:19

failed SlapOS.Eggs.UnitTest-Master.Python2

00:39:40

failed SlapOS.Eggs.UnitTest-Master.Python2

00:27:56

failed SlapOS.Eggs.UnitTest-Master.Python3

00:16:07

passed SlapOS.Eggs.UnitTest-Master.Python3

00:32:07

passed SlapOS.Eggs.UnitTest-Master.Python3

00:09:56

passed SlapOS.Eggs.UnitTest-Master.Python3

00:12:10

passed SlapOS.Eggs.UnitTest-Master.Python3

00:33:07