Don't attempt to optimize the space allocated for bpf headers if

sizeof(struct bpf_hdr) > 20.  20 is normal on 32-bit systems with
32-bit alignment, but we still assume that the last 2 bytes of the
struct are unnecessary padding on such systems.  On systems with
64-bit longs, struct timeval is bloated to 16 bytes, so bpf headers
certainly don't fit in 18 bytes.

sys/net/bpf.h

@@ -38,7 +38,7 @@
  *      @(#)bpf.h	8.1 (Berkeley) 6/10/93
  *	@(#)bpf.h	1.34 (LBL)     6/16/96
  *
- * $Id$
+ * $Id: bpf.h,v 1.12 1997/02/22 09:40:55 peter Exp $
  */
 
 #ifndef _NET_BPF_H_
@@ -128,7 +128,8 @@ struct bpf_hdr {
  * Only the kernel needs to know about it; applications use bh_hdrlen.
  */
 #ifdef KERNEL
-#define SIZEOF_BPF_HDR 18
+#define	SIZEOF_BPF_HDR	(sizeof(struct bpf_hdr) <= 20 ? 18 : \
+    sizeof(struct bpf_hdr))
 #endif
 
 /*