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>

include/linux/bpf_verifier.h

@@ -582,6 +582,7 @@ bpf_get_spilled_stack_arg(int slot, struct bpf_func_state *frame)
 				(void)(__expr);                          \
 			}						 \
 		}                                                        \
+		(void)__stack;                                           \
 	})
 
 /* Invoke __expr over regsiters in __vst, setting __state and __reg */