drm/dp_mst: Ensure mst_primary pointer is valid in drm_dp_mst_handle_up_req()

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

drm/dp_mst: Ensure mst_primary pointer is valid in drm_dp_mst_handle_up_req()

While receiving an MST up request message from one thread in

drm_dp_mst_handle_up_req(), the MST topology could be removed from

another thread via drm_dp_mst_topology_mgr_set_mst(false), freeing

mst_primary and setting drm_dp_mst_topology_mgr::mst_primary to NULL.

This could lead to a NULL deref/use-after-free of mst_primary in

drm_dp_mst_handle_up_req().

Avoid the above by holding a reference for mst_primary in

drm_dp_mst_handle_up_req() while it's used.

v2: Fix kfreeing the request if getting an mst_primary reference fails.