xen-netback: fix guest Rx stall detection (after guest Rx refactor)

If a VIF has been ready for rx_stall_timeout (60s by default) and an
Rx ring is drained of all requests an Rx stall will be incorrectly
detected.  When this occurs and the guest Rx queue is empty, the Rx
ring's event index will not be set and the frontend will not raise an
event when new requests are placed on the ring, permanently stalling
the VIF.

This is a regression introduced by eb1723a2

 (xen-netback:
refactor guest rx).

Fix this by reinstating the setting of queue->last_rx_time when
placing a packet onto the guest Rx ring.

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