accel/rocket: search every core slot when looking up a scheduler

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

accel/rocket: search every core slot when looking up a scheduler

sched_to_core() walks rdev->cores[] up to rdev->num_cores, and
rocket_remove() decrements num_cores for every core it removes. Unbind a
core that is not the last one and the cores behind it fall outside the
search, so sched_to_core() returns NULL for a core that is still bound and
still running jobs. Neither caller checks the result:

rocket_job_run(): rocket_fence_create(core), core->dev
rocket_job_timedout(): dev_err(core->dev, "NPU job timed out")

Unbinding the middle core of the three on an RK3588 while three clients are
submitting to all of them faults twice, once from the surviving core's
job queue and once from its reset work:

KASAN: null-ptr-deref in range [0x0000000000000220-0x0000000000000227]
Workqueue: fdad0000.npu drm_sched_run_job_work [gpu_sched]
pc : rocket_job_run+0x234/0x838 [rocket]
Call trace:
rocket_job_run+0x234/0x838 [rocket]
drm_sched_run_job_work+0x2cc/0xad8 [gpu_sched]
process_one_work+0x640/0x14f0

KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
Workqueue: rocket-reset-2 drm_sched_job_timedout [gpu_sched]
pc : rocket_job_timedout+0xf0/0x1e0 [rocket]
Call trace:
rocket_job_timedout+0xf0/0x1e0 [rocket]
drm_sched_job_timedout+0x188/0x6a0 [gpu_sched]

Both are the third core: the workqueue names are its device and its
core->index, and it was left at slot 2 while num_cores had dropped to 2.

Search all the slots that were allocated, the way find_core_for_dev() now
does. A core that is still bound is then found, and the two callers get
the pointer they already assume they have.

This does not make unbinding one core out of several safe. An open client
keeps an entity pointing at the scheduler of the core that went away:
drm_sched reports it as not ready for every job that lands on it, and the
client waits in dma_fence_default_wait for a fence that will never signal.
Stopping the NULL dereference is what belongs in a fix; the rest wants
more thought.