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

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 1650d5455bd2dc6b5ee134bd6fc1a3236c266b5b (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>