Include packet timestamp into the mbuf packet header

[Mike Belopuhov]

One of the prerequisites for FQ_CoDel is ability to track packet
enqueue time.  To avoid allocating per-packet mbuf tags, I'd prefer
to include the timestamp directly into the packet header structure.
This can be later used for other purposes as well if need be.  OK?

diff --git sys/sys/mbuf.h sys/sys/mbuf.h
index 202ce8ced8b..daa9facf0dd 100644
--- sys/sys/mbuf.h
+++ sys/sys/mbuf.h
@@ -128,10 +128,11 @@ struct    pkthdr {
        u_int16_t                csum_flags;    /* checksum flags */
        u_int16_t                ether_vtag;    /* Ethernet 802.1p+Q vlan tag */
        u_int                    ph_rtableid;   /* routing table id */
        u_int                    ph_ifidx;      /* rcv interface index */
        u_int8_t                 ph_loopcnt;    /* mbuf is looping in kernel */
+       struct timeval           ph_timestamp;  /* packet timestamp */
        struct pkthdr_pf         pf;
 };
 
 /* description of external storage mapped into mbuf, valid if M_EXT set */
 struct mbuf_ext {