MINOR: tools/rnd: compute the result outside of the CAS loop

MINOR: tools/rnd: compute the result outside of the CAS loop

ha_random64() uses a DWCAS loop to produce the random, but it computes
the resulting value inside the loop while it doesn't change upon success,
so this is a needless overhead inside the critcal path that participates
to making threads fail the race and try again. Let's take the value out
of the loop.