urcu: add lfht_iteration_adapter helper

urcu: add lfht_iteration_adapter helper

The urcu lfht macros often make use of caa_container_of (and other
equivalent variations) which use offsetof. Unfortunately, offsetof is
conditionally supported by compilers for non-POD types.

The tree already has lttng::utils::container_of to work around this
problem. This new utils makes it possible to iterate on all of the
elements of an lfht without using those macros. Those iterations are the
main reason such warnings are emitted. The interface of
lfht_iteration_adapter also allows the use of ranged-for loops.

Change-Id: I61906e025bd0dd7512f02180700f3ddb3c9cf3ca
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>