BUG/MEDIUM: sample: fix random number upper-bound

random() will generate a number between 0 and RAND_MAX. POSIX mandates
RAND_MAX to be at least 32767. GNU libc uses (1<<31 - 1) as
RAND_MAX.

In smp_fetch_rand(), a reduction is done with a multiply and shift to
avoid skewing the results. However, the shift was always 32 and hence
the numbers were not distributed uniformly in the specified range. We
fix that by dividing by RAND_MAX+1. gcc is smart enough to turn that
into a shift:

    0x000000000046ecc8 <+40>:    shr    $0x1f,%rax