Add minimal subvolume support for /.

Rational: current installation of rootfs to default subvolume
makes a post-install flat subvolume structure (recommended!)
unnecessarily difficult to accomplish.

debian/changelog

@@ -1,3 +1,10 @@
+partman-btrfs (53) UNRELEASED; urgency=medium
+
+  [ Nicholas D Steeves ]
+  * Add minimal subvolume support for /.
+
+ -- Nicholas D Steeves <nsteeves@gmail.com>  Mon, 09 Mar 2020 17:26:59 -0400
+
 partman-btrfs (52) unstable; urgency=medium
 
   * Team upload

fstab.d/btrfs

@@ -17,10 +17,14 @@ for dev in $DEVICES/*; do
 			[ -f "$id/mountpoint" ] || continue
 			mountpoint=$(cat $id/mountpoint)
 			# due to #249322, #255135, #258117:
-			if [ "$mountpoint" = /tmp ]; then
+			if [ "$mountpoint" = "/tmp" ]; then
 				rm -f $id/options/noexec
 			fi
-			options=$(get_mountoptions $dev $id)
+			if [ "$mountpoint" = "/" ]; then
+				options="$(get_mountoptions $dev $id),subvol=@rootfs"
+			else
+				options=$(get_mountoptions $dev $id)
+			fi
 			# There is no btrfs fsck
 			echo "$path" "$mountpoint" btrfs $options 0 0
 			;;

mount.d/btrfs

@@ -11,7 +11,14 @@ pass=$6
 
 case $type in
     btrfs)
-	mount -t btrfs ${options:+-o "$options"} $fs /target$mp || exit 1
+        if [ "$mp" = "/" ]; then
+            mount -t btrfs ${options:+-o "$options"} $fs /target/ || exit 1
+            btrfs subvolume create /target/@rootfs || exit 1
+            umount /target || exit 1
+            mount -t btrfs ${options:+-o "$options,subvol=@rootfs"} $fs /target || exit 1
+        else
+            mount -t btrfs ${options:+-o "$options"} $fs /target"$mp" || exit 1
+        fi
 	echo "umount /target$mp"
 	exit 0
 	;;