The driver stored the WLUN periph pointer without holding a reference,
so the pointer went stale when the pass(4) device went away. In
addition, ufshci_sim_send_ssu() released a reference that it had never
acquired.

Define a simple ownership rule. ufshci_sim_find_periph() acquires the
periph and returns it. The cache owns one reference. The controller
destructor drops it with cam_periph_release() before taking the SIM
lock, since the release takes the CAM device lock by itself.
ufshci_sim_send_ssu() acquires its own reference and releases it when
done. Reuse the cached periph instead of searching again, so the old
reference is not leaked.

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