LoongArch: BPF: Remove duplicated flags check

The check for (BPF_TRAMP_F_ORIG_STACK | BPF_TRAMP_F_SHARE_IPMODIFY) is
duplicated in __arch_prepare_bpf_trampoline(). Remove it.

While at it, make sure stack_size and nargs are initialized once.

Cc: stable@vger.kernel.org
Tested-by: Vincent Li <vincent.mc.li@gmail.com>
Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>