migration/multifd: Avoid the final FLUSH in complete()

migration/multifd: Avoid the final FLUSH in complete()

We always do the flush when finishing one round of scan, and during
complete() phase we should scan one more round making sure no dirty page
existed.  In that case we shouldn't need one explicit FLUSH at the end of
complete(), as when reaching there all pages should have been flushed.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Tested-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>