CVE-2026-74659: Linux Kernel Uninitialised Bytes on the Wire in Bridge MRP Test Frames

The Linux bridge implementation of Media Redundancy Protocol builds its periodic test frames on a socket buffer whose data area is not cleared. On the ring-manager branch, the sub-option TLV header is appended without writing its length field, and two alignment bytes are appended without clearing them.

Every MRP test frame from a ring manager leaves the kernel with three bytes of whatever was in that memory. The fix appends the header and padding with a single zeroing call.