LoongArch: BPF: Fix uninitialized symbol 'retval_off'

In __arch_prepare_bpf_trampoline(), retval_off is meaningful only when
save_ret is not 0, so the current logic is correct. But it may cause a
build warning:

arch/loongarch/net/bpf_jit.c:1547 __arch_prepare_bpf_trampoline() error: uninitialized symbol 'retval_off'.

So initialize retval_off unconditionally to fix it.

Cc: stable@vger.kernel.org
Fixes: f9b6b41 ("LoongArch: BPF: Add basic bpf trampoline support")
Closes: https://lore.kernel.org/r/202508191020.PBBh07cK-lkp@intel.com/
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

Author: chenhuacai

arch/loongarch/net/bpf_jit.c

@@ -1502,11 +1502,10 @@ static int __arch_prepare_bpf_trampoline(struct jit_ctx *ctx, struct bpf_tramp_i
 	stack_size += 16;
 
 	save_ret = flags & (BPF_TRAMP_F_CALL_ORIG | BPF_TRAMP_F_RET_FENTRY_RET);
-	if (save_ret) {
-		/* Save BPF R0 and A0 */
-		stack_size += 16;
-		retval_off = stack_size;
-	}
+	if (save_ret)
+		stack_size += 16; /* Save BPF R0 and A0 */
+
+	retval_off = stack_size;
 
 	/* Room of trampoline frame to store args */
 	nargs = m->nr_args;