urcu: add lfht_filtered_iteration_adapter helper

urcu: add lfht_filtered_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 the
elements of an lfht that match a given key without using those macros. Those iterations are the
main reason such warnings are emitted. The interface of
lfht_filtered_iteration_adapter also allows the use of ranged-for loops.

Change-Id: I9acd3fa9f6523de8006bc9107bcca6b6b654d6fa
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>