patch 9.0.0751: 'scrolloff' does not work well with 'smoothscroll'

Problem:    'scrolloff' does not work well with 'smoothscroll'.
Solution:   Make positioning the cursor a bit better.  Rename functions.
diff --git a/src/edit.c b/src/edit.c
index 1aaac62..555b83b 100644
--- a/src/edit.c
+++ b/src/edit.c
@@ -237,7 +237,7 @@
 	if (startln)
 	    Insstart.col = 0;
     }
-    Insstart_textlen = (colnr_T)linetabsize(ml_get_curline());
+    Insstart_textlen = (colnr_T)linetabsize_str(ml_get_curline());
     Insstart_blank_vcol = MAXCOL;
     if (!did_ai)
 	ai_col = 0;
@@ -2372,7 +2372,7 @@
 	    // Don't update the original insert position when moved to the
 	    // right, except when nothing was inserted yet.
 	    update_Insstart_orig = FALSE;
-	Insstart_textlen = (colnr_T)linetabsize(ml_get_curline());
+	Insstart_textlen = (colnr_T)linetabsize_str(ml_get_curline());
 
 	if (u_save_cursor() == OK)
 	{