tests: md: Increase size of MD partitions in test.

Commit c11a92751e chose some strangely
sized partitions for testing Linux MD devices.  The disks were 100M
however the four partitions only covered the first 10M of disk space
(with the rest being unallocated and unused in the test).

This worked until Linux 4.20 where the ~2M-sized partitions become too
small for a Linux MD device.

This commit changes the test to use four 20M-sized partitions which is
sufficient space to create the MD device with recent kernels.  I also
modified the partition allocation code to use explicit calculations
and variables, making it considerably clearer.

tests/md/test-mdadm.sh

@@ -1,6 +1,6 @@
 #!/bin/bash -
 # libguestfs
-# Copyright (C) 2011 Red Hat Inc.
+# Copyright (C) 2011-2018 Red Hat Inc.
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -25,6 +25,12 @@ skip_if_skipped
 
 rm -f mdadm-{1,2,3,4}.img
 
+# Partition boundaries.
+p1_start=$(( 1024*1024/512 )); p1_end=$(( 20*1024*1024/512 - 1 ))
+p2_start=$(( p1_end+1 ));      p2_end=$(( 40*1024*1024/512 - 1 ))
+p3_start=$(( p2_end+1 ));      p3_end=$(( 60*1024*1024/512 - 1 ))
+p4_start=$(( p3_end+1 ));      p4_end=$(( 80*1024*1024/512 - 1 ))
+
 guestfish <<EOF
 # Add four empty disks
 sparse mdadm-1.img 100M
@@ -35,25 +41,25 @@ run
 
 # Create lots of test partitions.
 part-init /dev/sda mbr
-part-add /dev/sda p 4096 8191
-part-add /dev/sda p 8192 12287
-part-add /dev/sda p 12288 16383
-part-add /dev/sda p 16384 20479
+part-add /dev/sda p $p1_start $p1_end
+part-add /dev/sda p $p2_start $p2_end
+part-add /dev/sda p $p3_start $p3_end
+part-add /dev/sda p $p4_start $p4_end
 part-init /dev/sdb mbr
-part-add /dev/sdb p 4096 8191
-part-add /dev/sdb p 8192 12287
-part-add /dev/sdb p 12288 16383
-part-add /dev/sdb p 16384 20479
+part-add /dev/sdb p $p1_start $p1_end
+part-add /dev/sdb p $p2_start $p2_end
+part-add /dev/sdb p $p3_start $p3_end
+part-add /dev/sdb p $p4_start $p4_end
 part-init /dev/sdc mbr
-part-add /dev/sdc p 4096 8191
-part-add /dev/sdc p 8192 12287
-part-add /dev/sdc p 12288 16383
-part-add /dev/sdc p 16384 20479
+part-add /dev/sdc p $p1_start $p1_end
+part-add /dev/sdc p $p2_start $p2_end
+part-add /dev/sdc p $p3_start $p3_end
+part-add /dev/sdc p $p4_start $p4_end
 part-init /dev/sdd mbr
-part-add /dev/sdd p 4096 8191
-part-add /dev/sdd p 8192 12287
-part-add /dev/sdd p 12288 16383
-part-add /dev/sdd p 16384 20479
+part-add /dev/sdd p $p1_start $p1_end
+part-add /dev/sdd p $p2_start $p2_end
+part-add /dev/sdd p $p3_start $p3_end
+part-add /dev/sdd p $p4_start $p4_end
 
 # RAID 1.
 md-create r1t1 "/dev/sda1 /dev/sdb1"