Add fakey 'deflateBound()' function to the in-kernel zlib routines

It's not the real deflateBound() in newer zlib libraries, partly because
the upcoming usage of it won't have the "stream" available, so we can't
have the same interfaces anyway.

include/linux/zlib.h

@@ -506,6 +506,11 @@ extern int zlib_deflateReset (z_streamp strm);
    stream state was inconsistent (such as zalloc or state being NULL).
 */
 
+static inline unsigned long deflateBound(unsigned long s)
+{
+	return s + ((s + 7) >> 3) + ((s + 63) >> 6) + 11;
+}
+
 extern int zlib_deflateParams (z_streamp strm, int level, int strategy);
 /*
      Dynamically update the compression level and compression strategy.  The