linuxkpi: Add list_for_each_rcu()

This is a variant of list_for_each() where the next item in the list
is read using rcu_dereference().

This is used by the i915 DRM driver starting with Linux 6.8.

Reviewed by: emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49375