ppp: Don't stop and restart queue on every TX packet

For every transmitted packet, ppp_start_xmit() will stop the netdev
queue and then, if appropriate, restart it. This causes the TX softirq
to run, entirely gratuitously.

This is "only" a waste of CPU time in the normal case, but it's actively
harmful when the PPP device is a TEQL slave — the wakeup will cause the
offending device to receive the next TX packet from the TEQL queue, when
it *should* have gone to the next slave in the list. We end up seeing
large bursts of packets on just *one* slave device, rather than using
the full available bandwidth over all slaves.

This patch fixes the problem by *not* unconditionally stopping the queue
in ppp_start_xmit(). It adds a return value from ppp_xmit_process()
which indicates whether the queue should be stopped or not.

It *doesn't* remove the call to netif_wake_queue() from
ppp_xmit_process(), because other code paths (especially from
ppp_output_wakeup()) need it there and it's messy to push it out to the
other callers to do it based on the return value. So we leave it in
place — it's a no-op in the case where the queue wasn't stopped, so it's
harmless in the TX path.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>