LinuxKPI: Add compiler barriers to list_for_each_entry_lockless macro

so this list-traversal primitive may safely run concurrently with the
_rcu list-mutation primitives such as list_add_rcu() as long as the
traversal is guarded by rcu_read_lock().

Do it by reusing the "list_for_each_entry_rcu" macro which does the same.
On Linux it implements some additional lockdep stuff which we skip.

Also move the macro to linux/rculist.h where it resides on Linux.

Reviewed by: hselasky
Differential revision: https://reviews.freebsd.org/D30795