drm/amdgpu/vcn: fix integer overflow in dec_msg buffer count check

In the Linux kernel, the following vulnerability has been resolved: drm/amdgpu/vcn: fix integer overflow in dec_msg buffer count check If the supplied msg[2] (num_buffers) is 0x3FFFFFFF, the expression 6 + num_buffers * 4 wraps to 2 and the bounds check passes, letting the parser loop far past the end of the message BO. Triggering it additionally requires a ~4GiB mapping so that msg[1] survives the earlier "header does not fit in BO" check. Rewrite the test in division form, which is overflow-free by construction. Also update the message to reflect that msg is invalid.

Where it's fixed