Sssd: sssd-kcm: local denial of service via excessive memory preallocation

A flaw was found in `sssd-kcm`. A local user or process able to connect to the `sssd-kcm` UNIX socket can exploit this vulnerability. By sending a large request length header and then stalling the connection, an attacker can cause the system to preallocate significant memory. This leads to memory exhaustion within the `sssd-kcm` responder, resulting in a Denial of Service (DoS) for affected deployments.