Re-order calls to put_gfn() around wait queue invocations

Re-order calls to put_gfn() around wait queue invocations

Since we use wait queues to handle potential ring congestion cases,
code paths that try to generate a mem event while holding a gfn lock
would go to sleep in non-preemptible mode.

Most such code paths can be fixed by simply postponing event generation until
locks are released.

Signed-off-by: Adin Scannell <adin@scannell.ca>
Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>