patch 8.1.2120: some MB_ macros are more complicated than necessary

Problem:    Some MB_ macros are more complicated than necessary. (Dominique
            Pelle)
Solution:   Simplify the macros.  Expand inline.
diff --git a/src/findfile.c b/src/findfile.c
index 414dc4e..ddb4d10 100644
--- a/src/findfile.c
+++ b/src/findfile.c
@@ -1337,8 +1337,8 @@
 	prev2 = prev1;
 	prev1 = c1;
 
-	i += MB_PTR2LEN(s1 + i);
-	j += MB_PTR2LEN(s2 + j);
+	i += mb_ptr2len(s1 + i);
+	j += mb_ptr2len(s2 + j);
     }
     return s1[i] == s2[j];
 }