xsk: Return error code if force_zc is set

If force_zc is set, we should exit out with an error, not fall back to
copy mode.

Fixes: 921b68692abb ("xsk: Enable sharing of dma mappings")
Reported-by: Hulk Robot <[e-mail]>
Signed-off-by: Zhang Changzhong <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Magnus Karlsson <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

net/xdp/xsk_buff_pool.c

···
 
 	if (!pool->dma_pages) {
 		WARN(1, "Driver did not DMA map zero-copy buffers");
+		err = -EINVAL;
 		goto err_unreg_xsk;
 	}
 	pool->umem->zc = true;