Abort udev upgrade if devtmpfs support is missing

Add preinst check to abort udev upgrade if the currently running kernel
lacks devtmpfs support. Since udev 176, devtmpfs is mandatory as udev no
longer creates any device nodes itself. This only affects self-compiled
kernels which now need CONFIG_DEVTMPFS=y.  Closes: #722580

debian/udev.preinst

@@ -117,6 +117,12 @@ check_kernel_features() {
 
   fi
 
+  local missing_devtmpfs=0
+  if ! grep -q '[[:space:]]devtmpfs$' /proc/filesystems; then
+    missing_devtmpfs=1
+    abort_install=1
+  fi
+
   local sysfs_deprecated=0
   if [ -d /sys/class/mem/null -a ! -L /sys/class/mem/null ]; then
     sysfs_deprecated=1
@@ -135,6 +141,15 @@ the running kernel:
 END
   fi
 
+  if [ "$missing_devtmpfs" -eq 1 ]; then
+    cat <<END
+Since release 176, udev requires support for the following features in
+the running kernel:
+
+- devtmpfs         (CONFIG_DEVTMPFS)
+END
+  fi
+
   if [ "$sysfs_deprecated" -eq 1 ]; then
     db_input critical udev/sysfs_deprecated_incompatibility || true
   fi
@@ -203,7 +218,7 @@ check_version() {
   # $2 is non-empty when installing from the "config-files" state
   [ "$2" ] || return 0
 
-  if dpkg --compare-versions $2 lt 168-2; then
+  if dpkg --compare-versions $2 lt 204-4; then
     # these must be checked first to allow aborting before changing anything
     if chrooted; then
       echo 'Running in a chroot, skipping the kernel versions checks!'