CVE-2026-5071: can: Local Denial of Service via SocketCAN Send

The SocketCAN implementation validates the length of a user-provided buffer containing a socketcanframe object using only a NETASSERT statement in zcansendtoctx() before dereferencing it in socketcantocanframe(). In production builds where assertions are disabled, a userspace application that controls the length passed to a sendto syscall can supply an incomplete or truncated frame, causing socketcantocanframe() to dereference fields beyond the end of the buffer. This results in an out-of-bounds read that can cause denial-of-service crashes or, because the parsed frame contents are transmitted on the network, leak adjacent memory.