xen-netback: use local var in xenvif_tx_check_gop() instead of re-calculating

shinfo already holds the result of skb_shinfo(skb) at this point - no
need to re-invoke the construct even twice.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>