accel/amdxdna: clear the mailbox channel pointer when starting it fails

aie2_create_context() allocates a mailbox channel, starts it, and on a
failed start frees it again:

	hwctx->priv->mbox_chann = xdna_mailbox_alloc_channel(ndev->mbox);
	...
	ret = xdna_mailbox_start_channel(hwctx->priv->mbox_chann, ...);
	if (ret)
		goto free_channel;
	...
free_channel:
	xdna_mailbox_free_channel(hwctx->priv->mbox_chann);

hwctx->priv->mbox_chann keeps pointing at the freed channel. That
pointer is the driver's own test for whether a hardware context has a
usable channel: aie2_destroy_context() returns early on NULL,
aie2_sched_job_run() refuses to run a job on NULL, and the message
helpers return -ENODEV on NULL. A stale pointer passes all of them.

It matters on the restart path. aie2_hwctx_restart() calls
aie2_create_context() again on a hardware context that is already live,
after aie2_hwctx_stop() has torn its channel down, and the context
survives a failure there: aie2_sched_job_timedout() discards the return
value entirely, and aie2_hwctx_resume() only propagates it. What is
left is a live hardware context holding a freed channel, and every
later user of it takes the non-NULL branch:

  - the next job submitted reaches xdna_mailbox_send_msg() on the freed
    channel, via the !mbox_chann guard in aie2_sched_job_run(),
  - a second command timeout reaches aie2_hwctx_stop() ->
    aie2_destroy_context(),
  - closing the device reaches aie2_hwctx_fini() ->
    aie2_release_resource() -> xrs_release_resource() ->
    aie2_xrs_unload() -> aie2_destroy_context(),

and aie2_destroy_context() then calls xdna_mailbox_stop_channel()
followed by xdna_mailbox_free_channel() on memory that was already
freed: a use-after-free, and a second free of the same channel.

The AIE4 management channel already does this correctly -
aie4_mailbox_start() clears ndev->aie.mgmt_chann right after freeing
it. Do the same here.

The create path itself is not affected: when aie2_xrs_load() fails,
xrs_allocate_resource() removes the solver node without calling
->unload, and aie2_hwctx_init() frees hwctx->priv, so the stale pointer
never outlives the structure holding it.

Starting a channel can fail today without this patch: the ring buffer
sizes firmware reports are rejected unless both are powers of two, and
request_irq() can fail.

Fixes: d5b8b0347fa8 ("accel/amdxdna: Split mailbox channel create function")
Cc: stable@vger.kernel.org
Signed-off-by: Eva Crystal <0xiviel@gmail.com>
Reviewed-by: Lizhi Hou <lizhi.hou@amd.com>
Signed-off-by: Lizhi Hou <lizhi.hou@amd.com>
Link: https://patch.msgid.link/2fcf0eca215090dbbd95cb86e1bf5076056d3818.1789334558.git.0xiviel@gmail.com

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