check_disk: die with a reasonable error message if one mount point hits average value

plugins/t/check_disk.t

@@ -57,6 +57,9 @@ if ($free_on_mp1 > $free_on_mp2) {
 } else {
 	die "Two mountpoints are the same - cannot do rest of test";
 }
+if($free_on_mp1 == $avg_free || $free_on_mp2 == $avg_free) {
+	die "One mountpoints has average space free - cannot do rest of test";
+}
 
 
 # Do same for inodes
@@ -74,6 +77,9 @@ if ($free_inode_on_mp1 > $free_inode_on_mp2) {
 } else {
 	die "Two mountpoints with same inodes free - cannot do rest of test";
 }
+if($free_inode_on_mp1 == $avg_inode_free || $free_inode_on_mp2 == $avg_inode_free) {
+	die "One mountpoints has average inodes free - cannot do rest of test";
+}
 
 # Verify performance data
 # First check absolute thresholds...