selftests/bpf: Test dynptr teardown on subprog return

Add failure tests for slices that escape a subprogram from a dynptr in the
callee stack. Cover immediate use of a ring buffer clone slice,
release-time invalidation of a caller dynptr slice after it crosses a
subprogram return, and immediate use of an skb dynptr slice in a qdisc
program.

Cover the error path where a subprogram would otherwise lose the last
dynptr capable of releasing a referenced resource.

Also add a success control which derives a slice from a caller-owned dynptr
and creates a separate clone in the callee. Destroying that callee-local
clone on return must not invalidate the caller dynptr's slice.

Signed-off-by: Xu Yunxiang <xyx2021@mail.ustc.edu.cn>
Reviewed-by: Amery Hung <ameryhung@gmail.com>
Link: https://lore.kernel.org/bpf/20260927120422.1042107-3-xyx2021@mail.ustc.edu.cn
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

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