Set _sub to "0" when _kver_split[2] is null

Fixes release kernel versioning, which is lacking the subversion. Example: kernel 6.0 release was versioned as "6.0." and will now be "6.0.0".
This also unifies versioning as it'll now always be x.x.x format.

linux-tkg-config/prepare

@@ -176,7 +176,11 @@ _set_kver_internal_vars() {
   _basever="${_kver_split[0]}${_kver_split[1]}"
 
   # examples: "5", "rc2", "122"
-  _sub="${_kver_split[2]}"
+  if [ -n "${_kver_split[2]}" ]; then
+    _sub="${_kver_split[2]}"
+  else
+    _sub="0"
+  fi
 
   # Append a zero to the minor if it has single digit
   [[ ${#_kver_split[1]} == "1" ]] && _kver_split[1]="0${_kver_split[1]}"