net/xen-netback: xenvif_gop_frag_copy: move GSO check out of the loop

The skb doesn't change within the function. Therefore it's only
necessary to check if we need GSO once at the beginning.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>