Use naming consistent with other common functions

Use naming consistent with other common functions

arc4random(3) returns a number.
arc4random_buf(3) fills a buffer.
arc4random_uniform(3) returns a number less than a bound.

and I'd add a hypothetical one which we use:

*_interval() should return a number within the interval [min, max].

In reality, the function being called csrand() in this patch is not
really cryptographically secure, since it had a bias, but a subsequent
patch will fix that.

Signed-off-by: Alejandro Colomar <alx@kernel.org>