drm/nouveau: fix a use-after-free in nouveau_gem_prime_import_sg_table()

drm/nouveau: fix a use-after-free in nouveau_gem_prime_import_sg_table()

nouveau_bo_init() is backed by ttm_bo_init() and ferries its return code
back to the caller. On failures, ttm will call nouveau_bo_del_ttm() and
free the memory.Thus, when nouveau_bo_init() returns an error, the gem
object has already been released. Then the call to nouveau_bo_ref() will
use the freed "nvbo->bo" and lead to a use-after-free bug.

We should delete the call to nouveau_bo_ref() to avoid the use-after-free.

Signed-off-by: Jianglei Nie <niejianglei2021@163.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Fixes: 019cbd4a4feb ("drm/nouveau: Initialize GEM object before TTM object")
Cc: Thierry Reding <treding@nvidia.com>
Cc: <stable@vger.kernel.org> # v5.4+
Link: https://patchwork.freedesktop.org/patch/msgid/20220705132546.2247677-1-niejianglei2021@163.com