workqueue: avoid unguarded 64-bit division

workqueue: avoid unguarded 64-bit division

The printk() requires a division that is not allowed on 32-bit architectures:

x86_64-linux-ld: lib/test_workqueue.o: in function `test_workqueue_init':
test_workqueue.c:(.init.text+0x36f): undefined reference to `__udivdi3'

Use div_u64() to print the resulting elapsed microseconds.

Fixes: 24b2e73f9700 ("workqueue: add test_workqueue benchmark module")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tejun Heo <tj@kernel.org>