xen-netback: use skb to determine number of required guest Rx requests

Using the MTU or GSO size to determine the number of required guest Rx
requests for an skb was subtly broken since these value may change at
runtime.

After 1650d545

 (xen-netback: always
fully coalesce guest Rx packets) we always fully pack a packet into
its guest Rx slots.  Calculating the number of required slots from the
packet length is then easy.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>