bpf: Silence unused-but-set-variable warning in bpf_for_each_reg_in_vstate_mask

The macro requires callers to pass a stack variable, but not all
callbacks use it. Add (void)__stack to suppress the clang W=1 warning.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Link: https://lore.kernel.org/r/20260602175204.624401-1-ameryhung@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>