xen-netback: notify immediately after pushing Tx response.

xen-netback: notify immediately after pushing Tx response.

This fixes a performance regression introduced by
7fbb9d8415d4a51cf542e87cf3a717a9f7e6aedc (xen-netback: release pending
index before pushing Tx responses)

Moving the notify outside of the spin locks means it can be delayed a
long time (if the dealloc thread is descheduled or there is an
interrupt or softirq).

Signed-off-by: David Vrabel <[email protected]>
Reviewed-by: Zoltan Kiss <[email protected]>
Acked-by: Wei Liu <[email protected]>
Signed-off-by: David S. Miller <[email protected]>