'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>