Fix directory detection in dkms.mkconf

Fix `zfs-dkms` installation on Debian-derived distributions by
aligning the directory detection logic to openzfs#13096.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: szubersk <szuberskidamian@gmail.com>
Closes openzfs#11449
Closes openzfs#13141

scripts/dkms.mkconf

@@ -29,22 +29,14 @@ PRE_BUILD="configure
   --prefix=/usr
   --with-config=kernel
   --with-linux=\$(
-    case \`lsb_release -is\` in
-      (Debian|Devuan)
-        if [[ -e \${kernel_source_dir/%build/source} ]]
-        then
-          echo \${kernel_source_dir/%build/source}
-        else
-          # A kpkg exception for Proxmox 2.0
-          echo \${kernel_source_dir}
-        fi
-      ;;
-      (*)
-        echo \${kernel_source_dir}
-      ;;
-    esac
+    if [ -e "\${kernel_source_dir/%build/source}" ]
+    then
+      echo "\${kernel_source_dir/%build/source}"
+    else
+      echo "\${kernel_source_dir}"
+    fi
   )
-  --with-linux-obj=\${kernel_source_dir}
+  --with-linux-obj="\${kernel_source_dir}"
   \$(
     [[ -n \"\${ICP_ROOT}\" ]] && \\
     {