[NET_SCHED]: Making rate table lookups more flexible.

This is done in order to, add support to changing the rate table to
use the upper-boundry L2T (length to time) value. Currently we use the
lower-boundry, which result in under-estimating the actual bandwidth
usage.

Extend the tc_ratespec struct, with two parameters: 1) "cell_align"
that allow adjusting the alignment of the rate table. 2) "overhead"
that allow adding a packet overhead before the lookup.
Signed-off-by: Jesper Dangaard Brouer <hawk@comx.dk>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/pkt_sched.h

@@ -77,8 +77,8 @@ struct tc_ratespec
 {
 	unsigned char	cell_log;
 	unsigned char	__reserved;
-	unsigned short	feature;
-	short		addend;
+	unsigned short	overhead;
+	short		cell_align;
 	unsigned short	mpu;
 	__u32		rate;
 };

include/net/sch_generic.h

@@ -307,7 +307,9 @@ drop:
  */
 static inline u32 qdisc_l2t(struct qdisc_rate_table* rtab, unsigned int pktlen)
 {
-	int slot = pktlen;
+	int slot = pktlen + rtab->rate.cell_align + rtab->rate.overhead;
+	if (slot < 0)
+		slot = 0;
 	slot >>= rtab->rate.cell_log;
 	if (slot > 255)
 		return (rtab->data[255]*(slot >> 8) + rtab->data[slot & 0xFF]);