Try to be tolerant of FWNMI delivery errors if the machine check had been
recovered by the host.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Message-Id: <20200325142906.221248-5-npiggin@gmail.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
[dwg: Updated comment at Greg's suggestion]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>