pktgen: add pktgen_handle_all_threads() for the same code

The pktgen_{run, reset, stop}_all_threads() has the same code,
so add pktgen_handle_all_threads() for it.

Signed-off-by: Yejune Deng <yejune.deng@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>