Move implementation of ioctls into kern_*() functions.

Move locks from outside ioctl to the individual implementations.

This is the first step of changing the implementations to act on a
kernel-internal request struct rather than on struct md_ioctl and to
removing the use of kern_ioctl in mountroot.

Reviewed by: cem, kib, markj (prior version)
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D14700