Allow the Disk I/O Storage Group scheduler to fallback to free space

The "Balanced Disk I/O" Storage Group scheduler was indeterminate when
the weights of two filesystems were equal.  This commit makes that
decision more determinate by allowing the scheduler to fall back to using
the amount of free space on the filesystems to make the decision when
two filesystems have the same weight.

Closes #10946.

Signed-off-by: Chris Pinkham <cpinkham@mythtv.org>

mythtv/programs/mythbackend/scheduler.cpp

@@ -4502,11 +4502,19 @@ static bool comp_storage_free_space(FileSystemInfo *a, FileSystemInfo *b)
     return false;
 }
 
-// prefer dirs with less weight (disk I/O) over dirs with more weight
+// prefer dirs with less weight (disk I/O) over dirs with more weight.
+// if weights are equal, prefer dirs with more absolute free space over less
 static bool comp_storage_disk_io(FileSystemInfo *a, FileSystemInfo *b)
 {
     if (a->getWeight() < b->getWeight())
+    {
         return true;
+    }
+    else if (a->getWeight() == b->getWeight())
+    {
+        if (a->getFreeSpace() > b->getFreeSpace())
+            return true;
+    }
 
     return false;
 }