PM: hibernate: flush swap writer after marking

Flush the swap writer after, not before, marking the files, to ensure the
signature is properly written.

Fixes: 6f612af5 ("PM / Hibernate: Group swap ops")
Signed-off-by: Laurent Badel <laurentbadel@eaton.com>
Cc: All applicable <stable@vger.kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>