Provide KPI for handling of rw/ro kernel text.

This is a pure syntax patch to create an interface to enable and later
restore write access to the kernel text and other read-only mapped
regions. It is in line with e.g. vm_fault_disable_pagefaults() by
allowing the nesting.

Discussed with: Peter Lei <peter.lei@ieee.org>
Reviewed by: jtl
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D14768