pktgen: fix error message with wrong function name

Error on calling kthread_create_on_node prints wrong function name,
kernel_thread.

Fixes: 94dcf29a ("kthread: use kthread_create_on_node()")
Signed-off-by: Leesoo Ahn <dev@ooseel.net>
Acked-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>