refperf: Work around 64-bit division

A 64-bit division was introduced in refperf, breaking compilation
on all 32-bit architectures:

kernel/rcu/refperf.o: in function `main_func':
refperf.c:(.text+0x57c): undefined reference to `__aeabi_uldivmod'

Fix this by using div_u64 to mark the expensive operation.

[ paulmck: Update primitive and format per Nathan Chancellor. ]
Fixes: bd5b16d6c88d ("refperf: Allow decimal nanoseconds")
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Valdis Klētnieks <valdis.kletnieks@vt.edu>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>