xen-netfront: release per-queue Tx and Rx resource when disconnecting

Since netfront may reconnect to a backend with a different number of
queues, all per-queue Rx and Tx resources (skbs and grant references)
should be freed when disconnecting.

Without this fix, the Tx and Rx grant refs are not released and
netfront will exhaust them after only a few reconnections.  netfront
will fail to connect when no free grant references are available.

Since all Rx bufs are freed and reallocated instead of reused this
will add some additional delay to the reconnection but this is
expected to be small compared to the time taken by any backend hotplug
scripts etc.

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