virtio-net: free xsk_buffs on error in virtnet_xsk_pool_enable()

The selftests added to our CI by Bui Quang Minh recently reveals
that there is a mem leak on the error path of virtnet_xsk_pool_enable():

unreferenced object 0xffff88800a68a000 (size 2048):
  comm "xdp_helper", pid 318, jiffies 4294692778
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace (crc 0):
    __kvmalloc_node_noprof+0x402/0x570
    virtnet_xsk_pool_enable+0x293/0x6a0 (drivers/net/virtio_net.c:5882)
    xp_assign_dev+0x369/0x670 (net/xdp/xsk_buff_pool.c:226)
    xsk_bind+0x6a5/0x1ae0
    __sys_bind+0x15e/0x230
    __x64_sys_bind+0x72/0xb0
    do_syscall_64+0xc1/0x1d0
    entry_SYSCALL_64_after_hwframe+0x77/0x7f

Acked-by: Jason Wang <jasowang@redhat.com>
Fixes: e9f3962 ("virtio_net: xsk: rx: support fill with xsk buffer")
Link: https://patch.msgid.link/20250430163836.3029761-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/virtio_net.c

@@ -5890,8 +5890,10 @@ static int virtnet_xsk_pool_enable(struct net_device *dev,
 
 	hdr_dma = virtqueue_dma_map_single_attrs(sq->vq, &xsk_hdr, vi->hdr_len,
 						 DMA_TO_DEVICE, 0);
-	if (virtqueue_dma_mapping_error(sq->vq, hdr_dma))
-		return -ENOMEM;
+	if (virtqueue_dma_mapping_error(sq->vq, hdr_dma)) {
+		err = -ENOMEM;
+		goto err_free_buffs;
+	}
 
 	err = xsk_pool_dma_map(pool, dma_dev, 0);
 	if (err)
@@ -5919,6 +5921,8 @@ static int virtnet_xsk_pool_enable(struct net_device *dev,
 err_xsk_map:
 	virtqueue_dma_unmap_single_attrs(rq->vq, hdr_dma, vi->hdr_len,
 					 DMA_TO_DEVICE, 0);
+err_free_buffs:
+	kvfree(rq->xsk_buffs);
 	return err;
 }