ip: silence udp zerocopy smatch false positive

extra_uref is used in __ip(6)_append_data only if uarg is set.

Smatch sees that the variable is passed to sock_zerocopy_put_abort.
This function accesses it only when uarg is set, but smatch cannot
infer this.

Make this dependency explicit.

Fixes: 52900d22 ("udp: elide zerocopy operation in hot path")
Signed-off-by: NWillem de Bruijn <willemb@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_output.c

@@ -1130,7 +1130,8 @@ static int __ip_append_data(struct sock *sk,
 error_efault:
 	err = -EFAULT;
 error:
-	sock_zerocopy_put_abort(uarg, extra_uref);
+	if (uarg)
+		sock_zerocopy_put_abort(uarg, extra_uref);
 	cork->length -= length;
 	IP_INC_STATS(sock_net(sk), IPSTATS_MIB_OUTDISCARDS);
 	refcount_add(wmem_alloc_delta, &sk->sk_wmem_alloc);

net/ipv6/ip6_output.c

@@ -1575,7 +1575,8 @@ static int __ip6_append_data(struct sock *sk,
 error_efault:
 	err = -EFAULT;
 error:
-	sock_zerocopy_put_abort(uarg, extra_uref);
+	if (uarg)
+		sock_zerocopy_put_abort(uarg, extra_uref);
 	cork->length -= length;
 	IP6_INC_STATS(sock_net(sk), rt->rt6i_idev, IPSTATS_MIB_OUTDISCARDS);
 	refcount_add(wmem_alloc_delta, &sk->sk_wmem_alloc);