Replace 2 jiffies with sysctl netdev_budget_usecs to enable softirq tuning

Constants used for tuning are generally a bad idea, especially as hardware
changes over time. Replace the constant 2 jiffies with sysctl variable
netdev_budget_usecs to enable sysadmins to tune the softirq processing.
Also document the variable.

For example, a very fast machine might tune this to 1000 microseconds,
while my regression testing 486DX-25 needs it to be 4000 microseconds on
a nearly idle network to prevent time_squeeze from being incremented.

Version 2: changed jiffies to microseconds for predictable units.

Signed-off-by: Matthew Whitehead <tedheadster@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

kernel/sysctl_binary.c

@@ -197,6 +197,7 @@ static const struct bin_table bin_net_core_table[] = {
 	{ CTL_INT,	NET_CORE_AEVENT_ETIME,	"xfrm_aevent_etime" },
 	{ CTL_INT,	NET_CORE_AEVENT_RSEQTH,	"xfrm_aevent_rseqth" },
 	{ CTL_INT,	NET_CORE_WARNINGS,	"warnings" },
+	{ CTL_INT,	NET_CORE_BUDGET_USECS,	"netdev_budget_usecs" },
 	{},
 };