xen-netback: fix fragments error handling in checksum_setup_ip()

Fix to return -EPROTO error if fragments detected in checksum_setup_ip().

Fixes: 1431fb31ecba ('xen-netback: fix fragment detection in checksum setup')
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>