rand: make randX_upto() do the end value increment

We should not do it in the caller, the functions that need
fixing are really rand32/64_upto() instead.

Also move a (now) misplaced comment.

Fixes: c6fc6d2ab2c2 ("rand: ensure that rand_between() can reach max value")
Signed-off-by: Jens Axboe <axboe@kernel.dk>