LoongArch: csum: Fix OoB access in IP checksum code for negative lengths

LoongArch: csum: Fix OoB access in IP checksum code for negative lengths

Commit 69e3a6aa6be2 ("LoongArch: Add checksum optimization for 64-bit
system") would cause an undefined shift and an out-of-bounds read.

Commit 8bd795fedb84 ("arm64: csum: Fix OoB access in IP checksum code
for negative lengths") fixes the same issue on ARM64.

Fixes: 69e3a6aa6be2 ("LoongArch: Add checksum optimization for 64-bit system")
Co-developed-by: Wentao Guan <guanwentao@uniontech.com>
Signed-off-by: Wentao Guan <guanwentao@uniontech.com>
Signed-off-by: Yuli Wang <wangyuli@uniontech.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>