selftests/bpf: Test bpf_kptr_xchg stashing of bpf_rb_root

There was some confusion amongst Meta sched_ext folks regarding whether
stashing bpf_rb_root - the tree itself, rather than a single node - was
supported. This patch adds a small test which demonstrates this
functionality: a local kptr with rb_root is created, a node is created
and added to the tree, then the tree is kptr_xchg'd into a mapval.

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20231204211722.571346-1-davemarchevsky@fb.com