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/ex_cmds.c b/src/ex_cmds.c
index 3c13b8d..7f41c79 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -256,7 +256,7 @@
 	;
     save = *last;
     *last = NUL;
-    len = linetabsize(line);		// get line length
+    len = linetabsize_str(line);	// get line length on screen
     if (has_tab != NULL)		// check for embedded TAB
 	*has_tab = (vim_strchr(first, TAB) != NULL);
     *last = save;