[IPV4]: Get rid of bogus __in_put_dev in pktgen

This patch gets rid of a bogus __in_dev_put() in pktgen.c.  This was
spotted by Suzanne Wood.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>