selftests/bpf: fix implementation of smp_mb()

selftests/bpf: fix implementation of smp_mb()

As BPF doesn't include any barrier instructions, smp_mb() is implemented
by doing a dummy value returning atomic operation. Such an operation
acts a full barrier as enforced by LKMM and also by the work in progress
BPF memory model.

If the returned value is not used, clang[1] can optimize the value
returning atomic instruction in to a normal atomic instruction which
provides no ordering guarantees.

Mark the variable as volatile so the above optimization is never
performed and smp_mb() works as expected.

[1] https://godbolt.org/z/qzze7bG6z

Fixes: 88d706ba7cc5 ("selftests/bpf: Introduce arena spin lock")
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Link: https://lore.kernel.org/r/20250710175434.18829-2-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>