Linux kernel nfsd proc registration error handling flaw

CVE-2025-22026 is a Linux kernel vulnerability in the NFS server (nfsd) initialization path. The issue is that nfsd_proc_stat_init() ignored the return value of svc_proc_register() when creating the nfsd procfs statistics entry. If svc_proc_register() failed, the failure was not propagated, leaving initialization in an inconsistent state. Later, when the kernel attempted to remove the proc entry during teardown, it could trigger a WARN because the procfile had never been created successfully. The fix changes nfsd_proc_stat_init() to return the same pointer/error semantics as svc_proc_register() and updates nfsd_net_init() to validate that result and fail nfsd network namespace construction on error. According to the provided content, svc_proc_register() can fail if a dentry cannot be allocated or if an identical dentry already exists; the latter is considered unlikely in this code path and is treated as -ENOMEM.