[PKTGEN]: Add try_to_freeze()

The pktgen module prevents suspend-to-disk.  Fix.

Acked-by: "Michal Piotrowski" <michal.k.k.piotrowski@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>