net: mctp: usb: fix race between urb completion and rx_retry cancellation

In the Linux kernel, the following vulnerability has been resolved:

net: mctp: usb: fix race between urb completion and rx_retry cancellation

It's possible that sequencing between setting ->stopped and cancelling

the rx_retry work (in ndo_stop) could leave us with an urb queued:

T1: ndo_stop T2: rx_retry_work

------------ ----------------

LD: ->stopped => false

ST: ->stopped <= true

usb_kill_urb()

mctp_usb_rx_queue()

usb_submit_urb()

cancel_delayed_work_sync()

That urb completion can then re-schedule rx_retry_work.

Strenghen the sequencing between the stop (preventing another requeue)

and the cancel by updating both atomically under a new rx lock. After

setting ->rx_stopped, and cancelling pending work, we know that the

requeue cannot occur, so all that's left is killing any pending urb.