xen/gntdev: fix unmap notification order

While working with Xen's libxenvchan library I have faced an issue with
unmap notifications sent in wrong order if both UNMAP_NOTIFY_SEND_EVENT
and UNMAP_NOTIFY_CLEAR_BYTE were requested: first we send an event channel
notification and then clear the notification byte which renders in the below
inconsistency (cli_live is the byte which was requested to be cleared on unmap):

[  444.514243] gntdev_put_map UNMAP_NOTIFY_SEND_EVENT map->notify.event 6
libxenvchan_is_open cli_live 1
[  444.515239] __unmap_grant_pages UNMAP_NOTIFY_CLEAR_BYTE at 14

Thus it is not possible to reliably implement the checks like
- wait for the notification (UNMAP_NOTIFY_SEND_EVENT)
- check the variable (UNMAP_NOTIFY_CLEAR_BYTE)
because it is possible that the variable gets checked before it is cleared
by the kernel.

To fix that we need to re-order the notifications, so the variable is first
gets cleared and then the event channel notification is sent.
With this fix I can see the correct order of execution:

[   54.522611] __unmap_grant_pages UNMAP_NOTIFY_CLEAR_BYTE at 14
[   54.537966] gntdev_put_map UNMAP_NOTIFY_SEND_EVENT map->notify.event 6
libxenvchan_is_open cli_live 0

Cc: stable@vger.kernel.org
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20211210092817.580718-1-andr2000@gmail.com
Signed-off-by: Juergen Gross <jgross@suse.com>

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