selftests/bpf: Reject legacy packet loads from callbacks

Add verifier coverage for the callback restriction on legacy packet
loads. Exercise BPF_LD_ABS directly in a bpf_loop callback and
BPF_LD_IND from a static subprogram called by the callback, ensuring that
callback context follows nested static calls.

Also exercise a callback which reaches BPF_LD_IND through a global
function and its static descendant. A sibling success case calls the same
global chain outside a callback, preserving support for ordinary global
packet loads. Existing success cases continue to cover loads from ordinary
static subprograms.

The failure cases expect the policy-specific rejection instead of reaching
the implicit-return path, triggering a verifier warning, or being accepted
through a function boundary.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260903214758.2727663-9-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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