subiquity: Use LazyUnmount=yes on memorymapped /usr/lib/modules.

Subiquity images mount /usr/lib/modules from a squashfs, which systemd
tries to unmount on shutdown, whilst they are still being in use. As
systemd-udevd kmod built-in's libkmod has modules.* files
memorymapped. This produces an warning on shutdown, flooding the
screen with messages as systemd switches to a more verbose
output. Specify LazyUnmount=yes on that mount, such that unmount call
succeeds without flooding the shutdown log.

debian/changelog

@@ -1,3 +1,15 @@
+livecd-rootfs (2.616) UNRELEASED; urgency=medium
+
+  * Subiquity images mount /usr/lib/modules from a squashfs, which systemd
+    tries to unmount on shutdown, whilst they are still being in use. As
+    systemd-udevd kmod built-in's libkmod has modules.* files
+    memorymapped. This produces an warning on shutdown, flooding the
+    screen with messages as systemd switches to a more verbose
+    output. Specify LazyUnmount=yes on that mount, such that unmount call
+    succeeds without flooding the shutdown log.
+
+ -- Dimitri John Ledkov <xnox@ubuntu.com>  Fri, 04 Oct 2019 19:35:56 +0100
+
 livecd-rootfs (2.615) eoan; urgency=medium
 
   * magic-proxy: dump proxy log to stdout on failure

live-build/ubuntu-server/hooks/033-kernel-bits.binary

@@ -67,8 +67,11 @@ prereqs) exit 0;;
 esac
 
 echo ${kernel_metapkg} > /run/kernel-meta-package
-mkdir -p \$rootmnt/lib/modules
+mkdir -p \$rootmnt/usr/lib/modules
-mount \$rootmnt/cdrom/casper/extras/modules.squashfs-$flavor \$rootmnt/lib/modules
+mount \$rootmnt/cdrom/casper/extras/modules.squashfs-$flavor \$rootmnt/usr/lib/modules
+mkdir -p /run/systemd/system/usr-lib-modules.mount.d
+echo '[Mount]' >> /run/systemd/system/usr-lib-modules.mount.d/lazy.conf
+echo 'LazyUnmount=yes' >> /run/systemd/system/usr-lib-modules.mount.d/lazy.conf
 EOF
 	chmod +x "$KERNEL_BITS_ROOT"/etc/initramfs-tools/scripts/init-bottom/live-server