patch 9.0.1026: type of w_last_topfill is wrong

Problem:    type of w_last_topfill is wrong.
Solution:   Use "int" instead of "linenr_T". (closes #11670)
diff --git a/src/structs.h b/src/structs.h
index cefe655..497a353 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -3616,10 +3616,10 @@
 				    // window
 #endif
 
-    // five fields that are only used when there is a WinScrolled autocommand
+    // six fields that are only used when there is a WinScrolled autocommand
     linenr_T	w_last_topline;	    // last known value for w_topline
 #ifdef FEAT_DIFF
-    linenr_T	w_last_topfill;	    // last known value for w_topfill
+    int		w_last_topfill;	    // last known value for w_topfill
 #endif
     colnr_T	w_last_leftcol;	    // last known value for w_leftcol
     colnr_T	w_last_skipcol;	    // last known value for w_skipcol