CVE-2025-21789 LoongArch: csum: Fix OoB access in IP checksum code for negative lengths

In the Linux kernel, the following vulnerability has been resolved: 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...