crypto/rand: apply the same max read size on Illumos as on Solaris

crypto/rand: apply the same max read size on Illumos as on Solaris

This case was missed in CL 370894, and masked by the lack of an
Illumos TryBot.

Fixes #52452.

Change-Id: I7cda193e33c11a9d04eb888fdb5ec9218e6ed1b5
Reviewed-on: https://go-review.googlesource.com/c/go/+/401294
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <valsorda@google.com>