drm/i915/ttm: fix uaf with lmem_userfault_list handling

In the fault handler, make sure we check if the BO maps lmem after
we schedule the migration, since the current resource might change from
lmem to smem, if the pages are in the non-cpu visible portion of lmem.
This then leads to adding the object to the lmem_userfault_list even
though the current resource is no longer lmem. If we then destroy the
object, the list might still contain a link to the now free object, since
we only remove it if the object is still in lmem.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/7469


Fixes: ad74457a ("drm/i915/dgfx: Release mmap on rpm suspend")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221107165414.56970-1-matthew.auld@intel.com


(cherry picked from commit 625b7446)
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>