drm/amdgpu: Fix use-after-free race in VM acquire

In the Linux kernel, the following vulnerability has been resolved: drm/amdgpu: Fix use-after-free race in VM acquire Replace non-atomic vm->process_info assignment with cmpxchg() to prevent race when parent/child processes sharing a drm_file both try to acquire the same VM after fork(). (cherry picked from commit c7c573275ec20db05be769288a3e3bb2250ec618)