test_bpf: Fix NULL vs IS_ERR() check in test_skb_segment()

The skb_segment() function returns error pointers on error.  It never
returns NULL.

Fixes: 76db8087 ("net: bpf: add a test for skb_segment in test_bpf module")
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Acked-by: NDaniel Borkmann <daniel@iogearbox.net>
Reviewed-by: NYonghong Song <yhs@fb.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

lib/test_bpf.c

@@ -6649,7 +6649,7 @@ static __init int test_skb_segment(void)
 	}
 
 	segs = skb_segment(skb, features);
-	if (segs) {
+	if (!IS_ERR(segs)) {
 		kfree_skb_list(segs);
 		ret = 0;
 		pr_info("%s: success in skb_segment!", __func__);