FreeBSD: Use the new freeuio() helper to free dynamically allocated UIOs (#16300)

This freeuio() interface was introduced to FreeBSD recently. For now
it simply calls free(), so this change has no effect. However, this
may not always be true, and in CheriBSD this change is required.

Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brooks Davis <brooks.davis@sri.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>