drm/i915/gvt: Prevent use-after-free in ppgtt_free_all_spt()

ppgtt_free_all_spt() iterates the radixtree as it is deleting it,
forgoing all protection against the leaves being freed in the process
(leaving the iter pointing into the void).

A minimal fix seems to be to use the available post_shadow_list to
decompose the tree into a list prior to destroying the radixtree.

Alerted by the sparse warnings:

drivers/gpu/drm/i915/gvt/gtt.c:757:9: warning: incorrect type in assignment (different address spaces)
drivers/gpu/drm/i915/gvt/gtt.c:757:9:    expected void **slot
drivers/gpu/drm/i915/gvt/gtt.c:757:9:    got void [noderef] <asn:4> **
drivers/gpu/drm/i915/gvt/gtt.c:757:9: warning: incorrect type in assignment (different address spaces)
drivers/gpu/drm/i915/gvt/gtt.c:757:9:    expected void **slot
drivers/gpu/drm/i915/gvt/gtt.c:757:9:    got void [noderef] <asn:4> **
drivers/gpu/drm/i915/gvt/gtt.c:758:45: warning: incorrect type in argument 1 (different address spaces)
drivers/gpu/drm/i915/gvt/gtt.c:758:45:    expected void [noderef] <asn:4> **slot
drivers/gpu/drm/i915/gvt/gtt.c:758:45:    got void **slot
drivers/gpu/drm/i915/gvt/gtt.c:757:9: warning: incorrect type in argument 1 (different address spaces)
drivers/gpu/drm/i915/gvt/gtt.c:757:9:    expected void [noderef] <asn:4> **slot
drivers/gpu/drm/i915/gvt/gtt.c:757:9:    got void **slot
drivers/gpu/drm/i915/gvt/gtt.c:757:9: warning: incorrect type in assignment (different address spaces)
drivers/gpu/drm/i915/gvt/gtt.c:757:9:    expected void **slot
drivers/gpu/drm/i915/gvt/gtt.c:757:9:    got void [noderef] <asn:4> **

This would also have been loudly warning if run through CI for the
invalid RCU dereferences.

Fixes: b6c126a3 ("drm/i915/gvt: Manage shadow pages with radix tree")
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Changbin Du <changbin.du@intel.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>