Add a pmap invalidate that doesn't call sched_pin.

When demoting DMAP pages curthread may be pointing to data within the
page we are demoting. Create a new invalidate that doesn't pin and use
it in the demote case.

As the demote has both interrupts disabled, and is within a critical section
this is safe from having the scheduler from switching to another CPU.

Reported by: loos
Reviewed by: loos
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D13955