selftests/bpf: Fix map_kptr test.

The compiler is optimizing out majority of unref_ptr read/writes, so the test
wasn't testing much. For example, one could delete '__kptr' tag from
'struct prog_test_ref_kfunc __kptr *unref_ptr;' and the test would still "pass".

Convert it to volatile stores. Confirmed by comparing bpf asm before/after.

Fixes: 2cbc469a ("selftests/bpf: Add C tests for kptr")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20230214235051.22938-1-alexei.starovoitov@gmail.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>