Fix detection of root device resize

If xvda is not partitioned, checking size of device mounted as root dev
doesn't detect xvda resize - it will have the new size, while the fs on
it not. Change to checking actual filesystem size.

Also, improve log message to include the size.

Fixes QubesOS/qubes-issues#4274

init/resize-rootfs-if-needed.sh

@@ -11,14 +11,16 @@ if [ "$(blockdev --getro /dev/xvda)" -eq "1" ]; then
     exit 0
 fi
 
-sysfs_root_dev="/sys/dev/block/$(mountpoint -d /)"
 sysfs_xvda="/sys/class/block/xvda"
 
 # if root filesystem use already (almost) the whole dis
 non_rootfs_data=$(( 250 * 1024 * 2 ))
+rootfs_size=$(df --output=size / | tail -n 1)
+# convert to 512-byte blocks
+rootfs_size=$(( rootfs_size * 2 ))
 if [ "$(cat "$sysfs_xvda/size")" -lt \
-       $(( non_rootfs_data + $(cat "$sysfs_root_dev/size") )) ]; then
-   echo "root filesystem already at the right size" >&2
+       $(( non_rootfs_data + rootfs_size )) ]; then
+   echo "root filesystem already at $rootfs_size blocks" >&2
    exit 0
 fi