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.

MFC after: 2 weeks
Sponsored by: Axcient
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D38842