selftests/bpf: Use bpf_dynptr_slice() to read file dynptr in leak test

selftests/bpf: Use bpf_dynptr_slice() to read file dynptr in leak test

use_file_dynptr_slice_after_put_file() reads the dynptr via
bpf_dynptr_data(), which always returns NULL for a read-only file
dynptr, making the example confusing. Switch to bpf_dynptr_slice(), the
correct read API for file dynptrs, and read (rather than write) the slice
since it is read-only. The test still fails as expected.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Amery Hung <ameryhung@gmail.com>
Link: https://lore.kernel.org/r/20260605202056.1780352-6-ameryhung@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>