s2disk: Stop using deprecated SNAPSHOT_PMOPS ioctl

The SNAPSHOT_PMOPS ioctl has been deprecated for a very long time
and it shouldn't be used by s2disk any more, except when running
over a kernel that doesn't support the SNAPSHOT_PLATFORM_SUPPORT
ioctl.  However, even on such an old kernel, the PMOPS_FINISH
option for SNAPSHOT_PMOPS doesn't lead to any useful action, so
we can simply stop using it.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>