ppp: set qdisc_tx_busylock to avoid LOCKDEP splat

ppp: set qdisc_tx_busylock to avoid LOCKDEP splat

If a qdisc is installed on a ppp device, its possible to get
a lockdep splat under stress, because nested dev_queue_xmit() can
lock busylock a second time (on a different device, so its a false
positive)

Avoid this problem using a distinct lock_class_key for ppp
devices.

Reported-by: Yanko Kaneti <[email protected]>
Tested-by: Yanko Kaneti <[email protected]>
Signed-off-by: Eric Dumazet <[email protected]>
Signed-off-by: David S. Miller <[email protected]>