pktgen: fix misuse of BUG_ON() in pktgen_thread_worker()

pktgen: fix misuse of BUG_ON() in pktgen_thread_worker()

pktgen create threads for all online cpus and bond these threads to
relevant cpu repecivtily. when this thread firstly be woken up, it
will compare cpu currently running with the cpu specified at the time
of creation and if the two cpus are not equal, BUG_ON() will take effect
causing panic on the system.
Notice that these threads could be migrated to other cpus before start
running because of the cpu hotplug after these threads have created. so the
BUG_ON() used here seems unreasonable and we can replace it with WARN_ON()
to just printf a warning other than panic the system.

Signed-off-by: Di Zhu <zhudi21@huawei.com>
Link: https://lore.kernel.org/r/20210125124229.19334-1-zhudi21@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>