LinuxKPI: add DEFINE_LOCK_GUARD_0 for rcu

This adds guard support for non-real-types like rcu locking meaning
that we need to keep the lock state separately ourselves. _T is still
special and needs to be updated. Given it may not be used it needs
an unused attribute (we are using the LinuxKPI maybe_unused which
indeed is more expressive in this case).

Sponsored by: The FreeBSD Foundation (initially)
Reviewed by: dumbbell
Differential Revision: https://reviews.freebsd.org/D52076

(cherry picked from commit 3f48a0fe53ca7e001852f4a406c1bdfab1cd2f6d)