drm/dp_mst: Fix resetting msg rx state after topology removal

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

drm/dp_mst: Fix resetting msg rx state after topology removal

If the MST topology is removed during the reception of an MST down reply

or MST up request sideband message, the

drm_dp_mst_topology_mgr::up_req_recv/down_rep_recv states could be reset

from one thread via drm_dp_mst_topology_mgr_set_mst(false), racing with

the reading/parsing of the message from another thread via

drm_dp_mst_handle_down_rep() or drm_dp_mst_handle_up_req(). The race is

possible since the reader/parser doesn't hold any lock while accessing

the reception state. This in turn can lead to a memory corruption in the

reader/parser as described by commit bd2fccac61b4 ("drm/dp_mst: Fix MST

sideband message body length check").

Fix the above by resetting the message reception state if needed before

reading/parsing a message. Another solution would be to hold the

drm_dp_mst_topology_mgr::lock for the whole duration of the message

reception/parsing in drm_dp_mst_handle_down_rep() and

drm_dp_mst_handle_up_req(), however this would require a bigger change.

Since the fix is also needed for stable, opting for the simpler solution

in this patch.