rhashtable-test: Fix 64bit division

A 64bit division went in unnoticed. Use do_div() to accomodate
non 64bit architectures.

Reported-by: kbuild test robot
Fixes: 1aa661f5 ("rhashtable-test: Measure time to insert, remove & traverse entries")
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>