6lowpan: fix lockdep splats

6lowpan: fix lockdep splats

When a device ndo_start_xmit() calls again dev_queue_xmit(),
lockdep can complain because dev_queue_xmit() is re-entered and the
spinlocks protecting tx queues share a common lockdep class.

Same issue was fixed for bonding/l2tp/ppp in commits

3c0b2bce4c930 ("[PATCH] bonding: lockdep annotation")
122f7575ea83c ("bonding: set qdisc_tx_busylock to avoid LOCKDEP splat")
e1fe257a5f89b ("net: qdisc busylock needs lockdep annotations ")
83209e0c92d28 ("ppp: set qdisc_tx_busylock to avoid LOCKDEP splat ")

Reported-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Tested-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>