xen-netback: fix some error return code

'err' is overwrited to 0 after maybe_pull_tail() call, so the error
code was not set if skb_partial_csum_set() call failed. Fix to return
error -EPROTO from those error handling case instead of 0.

Fixes: d52eb0d46f36 ('xen-netback: make sure skb linear area covers checksum field')
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netback/netback.c

@@ -1209,8 +1209,10 @@ static int checksum_setup_ip(struct xenvif *vif, struct sk_buff *skb,
 			goto out;
 
 		if (!skb_partial_csum_set(skb, off,
-					  offsetof(struct tcphdr, check)))
+					  offsetof(struct tcphdr, check))) {
+			err = -EPROTO;
 			goto out;
+		}
 
 		if (recalculate_partial_csum)
 			tcp_hdr(skb)->check =
@@ -1227,8 +1229,10 @@ static int checksum_setup_ip(struct xenvif *vif, struct sk_buff *skb,
 			goto out;
 
 		if (!skb_partial_csum_set(skb, off,
-					  offsetof(struct udphdr, check)))
+					  offsetof(struct udphdr, check))) {
+			err = -EPROTO;
 			goto out;
+		}
 
 		if (recalculate_partial_csum)
 			udp_hdr(skb)->check =
@@ -1350,8 +1354,10 @@ static int checksum_setup_ipv6(struct xenvif *vif, struct sk_buff *skb,
 			goto out;
 
 		if (!skb_partial_csum_set(skb, off,
-					  offsetof(struct tcphdr, check)))
+					  offsetof(struct tcphdr, check))) {
+			err = -EPROTO;
 			goto out;
+		}
 
 		if (recalculate_partial_csum)
 			tcp_hdr(skb)->check =
@@ -1368,8 +1374,10 @@ static int checksum_setup_ipv6(struct xenvif *vif, struct sk_buff *skb,
 			goto out;
 
 		if (!skb_partial_csum_set(skb, off,
-					  offsetof(struct udphdr, check)))
+					  offsetof(struct udphdr, check))) {
+			err = -EPROTO;
 			goto out;
+		}
 
 		if (recalculate_partial_csum)
 			udp_hdr(skb)->check =