CLEANUP: ot: use the item API for the variables trees

CLEANUP: ot: use the item API for the variables trees

In flt_ot_vars_scope_dump(), switched from cebu64_first()/cebu64_next() to
cebu64_imm_first()/cebu64_imm_next() for iterating the variable name trees.
Since this function only reads variables under a read lock, the immutable
traversal API is the correct choice.  Also updated the container_of()
member from 'node' to 'name_node' to match the current struct var layout.