thunderbolt: Don't access a DP tunnel after its DPRX read was canceled

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

thunderbolt: Don't access a DP tunnel after its DPRX read was canceled

tb_dp_dprx_work() checks ->dprx_canceled before it takes tb->lock so it
misses a tb_dp_dprx_stop() that could not cancel the already running
work. It then polls the DPRX capabilities and runs the callback for a
tunnel that has already been torn down while the domain is suspending or
going away.

This can be hit by cancelling the DPRX read from outside the ordered
tb->wq: During suspend tb_disconnect_and_release_dp() does just this and
with a later patch tb_stop() will do it as well. The latter in combination
with the Apple NHI where the DPRX read never completed is how I hit
this.

Check the flag with tb->lock held instead and check it again in
tb_dp_tunnel_active() because the callback runs after the lock has been
dropped again.

Also clear the flag in tb_dp_dprx_start() so that it only ever describes
the work that is currently in flight.