radix_tree: add radix_tree_lookup/insert_ptr

radix_tree: add radix_tree_lookup/insert_ptr

Add simple 'wrapper' inline functions to insert or return ptr lookup value.
(So the user doesn't need to deal with 'union radix_value' locally and
also it makes easier to translage some lvm2 functions to use radix_tree).

Note: If the stored 'value' would NULL, it cannot be recognized
from a case of 'not found'. So usable only when 'values' stored with
tree are not NULL.