Backout my wrong attempt to fix the compilation warning in ip_input.c

and instead reapply the revision 1.49 of mbuf.h, i.e.

Fixed regression of the type of the `header' member of struct pkthdr from
`void *' to caddr_t in rev.1.51.  This mainly caused an annoying warning
for compiling ip_input.c.

Requested by:	bde

sys/netinet/ip_input.c

@@ -668,7 +668,7 @@ found:
 		 */
 		if (mff || ip->ip_off) {
 			ipstat.ips_fragments++;
-			m->m_pkthdr.header = (caddr_t)ip;
+			m->m_pkthdr.header = ip;
 #ifdef IPDIVERT
 			m = ip_reass(m,
 			    fp, &ipq[sum], &divert_info, &divert_cookie);

sys/sys/mbuf.h

@@ -92,7 +92,7 @@ struct pkthdr {
 	struct	ifnet *rcvif;		/* rcv interface */
 	int	len;			/* total packet length */
 	/* variables for ip and tcp reassembly */
-	caddr_t	header;			/* pointer to packet header */
+	void	*header;		/* pointer to packet header */
 	/* variables for hardware checksum */
 	int	csum_flags;		/* flags regarding checksum */
 	int	csum_data;		/* data field used by csum routines */