Use df instead of fdisk

some disk types do not contain proper partition tables like on time4vps
the type is simfs. On those fdisk fails to access the partition table,
thus being unable to determine the size of the volume.
df does only return the real usable disk space by the user, thus we
lower the 20GB threshold to 18

Fixes #275

setup/start/cloudron-resize-fs.sh

@@ -16,8 +16,8 @@ existing_swap=$(cat /proc/meminfo | grep SwapTotal | awk '{ printf "%.0f", $2/10
 readonly physical_memory=$(LC_ALL=C free -m | awk '/Mem:/ { print $2 }')
 readonly swap_size=$((${physical_memory} - ${existing_swap})) # if you change this, fix enoughResourcesAvailable() in client.js
 readonly app_count=$((${physical_memory} / 200)) # estimated app count
-readonly disk_size_bytes=$(LC_ALL=C fdisk -l ${disk_device} | grep "Disk ${disk_device}" | awk '{ printf $5 }') # can't rely on fdisk human readable units, using bytes instead
-readonly disk_size=$((${disk_size_bytes}/1024/1024))
+readonly disk_size_bytes=$(LC_ALL=C df | grep "${disk_device}" | awk '{ printf $2 }')
+readonly disk_size=$((${disk_size_bytes}/1024))
 readonly system_size=10240 # 10 gigs for system libs, apps images, installer, box code, data and tmp
 readonly ext4_reserved=$((disk_size * 5 / 100)) # this can be changes using tune2fs -m percent /dev/vda1