Patch from Eric Norum <eric@cls.usask.ca> to address the following problem

report from Philip A. Prindeville <philipp@zembu.com>:

        I was working on a device driver for a certain ethernet chipset that
    occassionally wraps in its buffer, and causes a resulting mbuf chain
    with only a few dozen bytes in the first mbuf of the chain.

        I wouldn't have thought this would be a problem, until I ran some
    stress tests that flooded the ethernet receiver with packets and
    started to get panics here:

       250
       251          if (m->m_pkthdr.len < sizeof(struct ip))
       252                  goto tooshort;
       253
       254  #ifdef  DIAGNOSTIC
       255          if (m->m_len < sizeof(struct ip))
       256                  panic("ipintr mbuf too short");
       257  #endif
       258
       259          if (m->m_len < sizeof (struct ip) &&
       260              (m = m_pullup(m, sizeof (struct ip))) == 0) {
       261                  ipstat.ips_toosmall++;
       262                  return;
       263          }
       264          ip = mtod(m, struct ip *);

    and the panic was at line 256.  But if I #undef'd DIAGNOSTICS,
    then the m_pullup() at line 260 does the right thing and the packet
    ends up being processed just fine.

    So I started wondering, (a) why was the test checking for
    something that apparently wasn't a fatal condition but rather
    one that is subsequently recovered from a couple of lines later
    and (b) why panic as a diagnostic "aid" from a recoverable
    condition rather than just (say) log a message to the console?

    All of this seems overly severe for no reason that is readily
    apparent to me.

c/src/exec/libnetworking/netinet/ip_input.c

@@ -251,7 +251,7 @@ ip_input(struct mbuf *m)
 	if (m->m_pkthdr.len < sizeof(struct ip))
 		goto tooshort;
 
-#ifdef	DIAGNOSTIC
+#if defined(DIAGNOSTIC) && defined(ORIGINAL_FREEBSD_CODE)
 	if (m->m_len < sizeof(struct ip))
 		panic("ipintr mbuf too short");
 #endif

c/src/libnetworking/netinet/ip_input.c

@@ -251,7 +251,7 @@ ip_input(struct mbuf *m)
 	if (m->m_pkthdr.len < sizeof(struct ip))
 		goto tooshort;
 
-#ifdef	DIAGNOSTIC
+#if defined(DIAGNOSTIC) && defined(ORIGINAL_FREEBSD_CODE)
 	if (m->m_len < sizeof(struct ip))
 		panic("ipintr mbuf too short");
 #endif

cpukit/libnetworking/netinet/ip_input.c

@@ -251,7 +251,7 @@ ip_input(struct mbuf *m)
 	if (m->m_pkthdr.len < sizeof(struct ip))
 		goto tooshort;
 
-#ifdef	DIAGNOSTIC
+#if defined(DIAGNOSTIC) && defined(ORIGINAL_FREEBSD_CODE)
 	if (m->m_len < sizeof(struct ip))
 		panic("ipintr mbuf too short");
 #endif