Add a way to inject fences using IPIs

A variant of this facility was already used by rmlocks where IPIs would
enforce ordering.

This allows to elide fences where they are rarely needed and the cost of
IPI (should it be necessary) is cheaper.

Reviewed by: kib, jeff (previous version)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21740