accel/amdxdna: reject a command chain that carries no commands

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

accel/amdxdna: reject a command chain that carries no commands

A chain whose command_count is zero passes the payload length check,
because struct_size(payload, data, 0) is just the header. The fill loop
then does not run, so offset stays zero and the request is submitted with
a zero-length buffer.

On firmware without AIE2_NPU_COMMAND that ends at the opcode check, since
op is still ERT_INVALID_CMD and aie2_get_chain_msg_op() answers
MSG_OP_MAX_OPCODE. aie2_get_npu_chain_msg_op() answers
MSG_OP_CHAIN_EXEC_NPU whatever it is given, so there the submission
continues to drm_clflush_virt_range(cmd_buf, 0), which reads the byte
before the buffer and faults on the vmap guard page. EXEC_CMD is
reachable by any process that can open the render node.

Reject the request instead.