Fix kernel memory disclosures in mpr and mps

In every mpr and mps ioctl that copies kernel data to userland, validate
that the requested length does not exceed the size of the kernel's
buffer.

Note that all of these ioctls already required root access.

Sponsored by: Axcient
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D38842

(cherry picked from commit 72aad3f9028af12e6c56a3a461b46a153abd7b24)