drm/i915: Avoid potential vm use-after-free

drm/i915: Avoid potential vm use-after-free

Adding the vm to the vm_xa table makes it visible to userspace, which
could try to race with us to close the vm.  So we need to take our extra
reference before putting it in the table.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Fixes: 9ec8795e7d91 ("drm/i915: Drop __rcu from gem_context->vm")
Cc: <stable@vger.kernel.org> # v5.16+
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230119173321.2825472-1-robdclark@gmail.com
(cherry picked from commit 99343c46d4e2b34c285d3d5f68ff04274c2f9fb4)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>