drm/nouveau/nouveau_bo: fix potential memory leak in nouveau_bo_alloc()

nouveau_bo_alloc() allocates a memory chunk for "nvbo" with kzalloc().
When some error occurs, "nvbo" should be released. But when
WARN_ON(pi < 0)) equals true, the function return ERR_PTR without
releasing the "nvbo", which will lead to a memory leak.

We should release the "nvbo" with kfree() if WARN_ON(pi < 0)) equals true.

Signed-off-by: Jianglei Nie <niejianglei2021@163.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220705094306.2244103-1-niejianglei2021@163.com