CVE-2026-72351: gue: validate REMCSUM private option length

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

gue: validate REMCSUM private option length

GUE private flags can indicate that remote checksum offload metadata is present. The private flags field itself is accounted for by guehdrflagslen(), but guehdrprivflagslen() currently returns 0 even when GUEPFLAGREMCSUM is set.

This lets a packet with only the private flags field pass validategueflags(), after which gueremcsum() and guegroremcsum() read the missing REMCSUM start/offset fields from the following bytes.

Account for GUEPLENREMCSUM when GUEPFLAGREMCSUM is present so that malformed packets are rejected during option validation.