selftests/bpf: rbtree: Fix incorrect global variable usage

Within __add_three() function, should use function parameters instead of
global variables. So that the variables groot_nested.inner.root and
groot_nested.inner.glock in rbtree_add_nodes_nested() are tested
correctly.

Signed-off-by: Rong Tao <rongtao@cestc.cn>
Link: https://lore.kernel.org/r/tencent_3DD7405C0839EBE2724AC5FA357B5402B105@qq.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>