FreeBSD: Sort out kernel FPU headers for 12.1-REL

We were missing an include for kernel FPU functions, breaking the build
on FreeBSD 12.1-RELEASE. This was apparently being pulled in from
elsewhere on stable/12 and head.

Sorted the other includes in these files while here.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11005