selftests/bpf: Check rbtree callback restrictions in subprogs

Add a verifier failure case where an rbtree comparator enters two nested
static subprograms and the innermost subprogram unlocks and relocks the
tree. Restoring the lock keeps the surrounding callback state balanced,
so the test specifically exercises whether the callback restriction follows
the nested calls.

Also add a load-only positive control whose comparator calls a harmless
static subprogram. This preserves the intended support for verified static
subprogram calls while holding the tree lock.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260903214758.2727663-3-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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