[PKT_SCHED]: Add jitter support to netem.

This patch adds jitter if desired to the delayed packets in the
netem scheduler.  I dropped the rate stuff out and reorganized so
that an underlying pfifo queue is used (next plan is to make it
have class ops).

The jitter is given as sigma to a Gaussian normal distribution.  The actual
implementation is a reduced form of the table driven stuff in NISTnet
(free).
Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@redhat.com>

include/linux/pkt_sched.h

@@ -409,6 +409,6 @@ struct tc_netem_qopt
 	__u32	loss;		/* random packet loss (0=none ~0=100%) */
 	__u32	gap;		/* re-ordering gap (0 for delay all) */
 	__u32   duplicate;	/* random packet dup  (0=none ~0=100%) */
-	__u32	rate;		/* maximum transmit rate (bytes/sec) */
+	__u32	jitter;		/* random jitter in latency (us) */
 };
 #endif