FreeBSD: fix compilation of FreeBSD world after 29274c9f6

prng32_bounded() is available to kernel only on FreeBSD 13+.

Call inline random_get_pseudo_bytes() with correct pointer type.
To be consistent, apply to Linux as well.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Martin Matuska <mm@FreeBSD.org>
Closes #12282