Comparing an offset from an object with NULL is undefined behavior
and the compiler may assume that this is never true.
This is indeed what is observed with gcc-10 miscompiling
cds_hlist_for_each_entry_rcu_2().
Fix this by introducing cds_hlist_entry_safe() rather than open-coding
the NULL check comparisons, and move cds_hlist_for_each_entry_2()
and cds_hlist_for_each_entry_safe_2() to this scheme as well.
Fixes: #1308
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Ief3531cf04e54b6dae05eb28a6822adfa141fdeb