xen-netback: process guest rx packets in batches

xen-netback: process guest rx packets in batches

Instead of only placing one skb on the guest rx ring at a time, process
a batch of up-to 64.  This improves performance by ~10% in some tests.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
[re-based]
Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 98f6d57ced73b723551568262019f1d6c8771f20)
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Shannon Nelson <shannon.nelson@oracle.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>