Call udevadm settle before kpartx -d

Back in 2017 some code was added to ignore failures tearing down loop
devices. But debugging that growpart race on cloud images made me (very)
aware of a potential cause of the race: doing something like zerofree on
a device will cause udev scripts to run, and if they are still running
by the time kpartx is called, you would expect the kpartx -d to fail. So
lets see if a udevadm settle helps, and get rid of one of the "sometimes
this fails but we don't know why" comments...

debian/changelog

@@ -1,3 +1,9 @@
+livecd-rootfs (2.656) UNRELEASED; urgency=medium
+
+  * Call udevadm settle before kpartx -d.
+
+ -- Michael Hudson-Doyle <michael.hudson@ubuntu.com>  Sat, 21 Mar 2020 00:07:43 +1300
+
 livecd-rootfs (2.655) focal; urgency=medium
 
   * minimized: reinstall packages one by one, instead of all of them

live-build/functions

@@ -14,18 +14,12 @@ clean_loops() {
     local kpartx_stdout
 
     if [ -n "${backing_img}" ]; then
-        # sync before removing loop to avoid "Device or resource busy" errors
+        # If something just finished writing to the device or a
+        # partition (e.g. the zerofree in umount_partition) udev might
+        # still be processing the device.
+        udevadm settle
         sync
-        kpartx_ret=""
+        kpartx -v -d "${backing_img}"
-        kpartx_stdout=$(kpartx -v -d "${backing_img}") || kpartx_ret=$?
-        echo "$kpartx_stdout"
-        if [ -n "$kpartx_ret" ]; then
-            if echo "$kpartx_stdout" | grep -q "loop deleted"; then
-                echo "Suppressing kpartx returning error (#860894)"
-            else
-                exit $kpartx_ret
-            fi
-        fi
         unset backing_img
     fi