LoongArch: BPF: Fix uninitialized symbol 'retval_off'

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: f9b6b41f0cf3 ("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>