Use a smarter fix for bug 1203.

Previously, we had incremented rand_bw so that when we later tested
"tmp >= rand_bw", we wouldn't have an off-by-one error.  But instead,
it makes more sense to leave rand_bw alone and test "tmp > rand_bw".

Note that this is still safe.  To take the example from the bug1203
writeup: Suppose that we have 3 nodes with bandwidth 1.  So the
bandwidth array is { 1, 1, 1 }, and the total bandwidth is 3.  We
choose rand_bw == 0, 1, or 2.  With the first iteration of the loop,
tmp is now 1; with the second, tmp is 2; with the third, tmp is 3.
Now that our check is tmp > rand_bw, we will set i in the first
iteration of the loop iff rand_bw == 0; in the second iteration of
the loop iff rand_bw == 1, and in the third iff rand_bw == 2.
That's what we want.

Incidentally, this change makes the bug 6538 fix more ironclad: once
rand_bw is set to UINT64_MAX, tmp > rand_bw is obviously false
regardless of the value of tmp.