xen-netback: use skb_is_gso in xenvif_start_xmit

xen-netback: use skb_is_gso in xenvif_start_xmit

In 5bd076708 ("Xen-netback: Fix issue caused by using gso_type wrongly")
we use skb_is_gso to determine if we need an extra slot to accommodate
the SKB. There's similar error in interface.c. Change that to use
skb_is_gso as well.

Signed-off-by: Wei Liu <[email protected]>
Cc: Annie Li <[email protected]>
Cc: Ian Campbell <[email protected]>
Cc: Paul Durrant <[email protected]>
Acked-by: Ian Campbell <[email protected]>
Signed-off-by: David S. Miller <[email protected]>