Docs/RCU/rculist_nulls: Fix hlist_[nulls]_head field names of 'obj'

The example code snippets on rculist_nulls.rst are assuming 'obj' to
have the 'hlist_head' or 'hlist_nulls_head' field named 'obj_node', but
a sentence and some code snippets are wrongly calling
'obj->obj_node.next' as 'obj->obj_next', or 'obj->obj_node' as 'member'.
Fix it.

Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>