selftests/bpf: Test kfunc returning zero-sized allocation buffer

Add a test passing an explicit rdwr_buf_size of 0 to
bpf_kfunc_call_test_get_rdwr_mem() and then reading the returned R0.
R0 should be a zero-sized PTR_TO_MEM, so the access must be rejected
with "min value is outside of the allowed memory range".

This covers the pre-existing bug where a zero size argument was
treated as "no size argument": the verifier fell through to
btf_resolve_size() and sized R0 after the pointed-to return type,
wrongly allowing the read.

Suggested-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Amery Hung <ameryhung@gmail.com>
Link: https://lore.kernel.org/bpf/20260715064047.1793790-6-ameryhung@gmail.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

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