xen-netback: immediately wake tx queue when guest rx queue has space

xen-netback: immediately wake tx queue when guest rx queue has space

When an skb is removed from the guest rx queue, immediately wake the
tx queue, instead of after processing them.

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 7c0b1a23e6f983fe392c8ffa71d05189ae52ebb5)
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>