patch 9.1.0218: Unnecessary multiplications in backspace code

Problem:  Unnecessary multiplications in backspace code, as
          "col / ts * ts" is the same as "col - col % ts".
Solution: Change "col / ts * ts" to "col - col % ts".  Adjust the loop
          and the comments ins_bs() to be easier to understand.  Update
          tests to reset 'smarttab' properly.
          (zeertzjq)

closes: #14308

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/indent.c b/src/indent.c
index 56032fa..1dfde7d 100644
--- a/src/indent.c
+++ b/src/indent.c
@@ -163,7 +163,7 @@
     int		excess;
 
     if (vts == NULL || vts[0] == 0)
-	return (col / ts) * ts;
+	return col - col % ts;
 
     tabcount = vts[0];
     for (t = 1; t <= tabcount; ++t)
@@ -174,7 +174,7 @@
     }
 
     excess = tabcol % vts[tabcount];
-    return excess + ((col - excess) / vts[tabcount]) * vts[tabcount];
+    return col - (col - excess) % vts[tabcount];
 }
 
 /*