net: pktgen: don't abuse current->state in pktgen_thread_worker()

Commit 1fbe4b46 "net: pktgen: kill the Wait for kthread_stop
code in pktgen_thread_worker()" removed (in particular) the final
__set_current_state(TASK_RUNNING) and I didn't notice the previous
set_current_state(TASK_INTERRUPTIBLE). This triggers the warning
in __might_sleep() after return.

Afaics, we can simply remove both set_current_state()'s, and we
could do this a long ago right after ef87979c "pktgen: better
scheduler friendliness" which changed pktgen_thread_worker() to
use wait_event_interruptible_timeout().

Reported-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>