sunrpc: harden rq_procinfo lifecycle to prevent double-free

Summary

In the Linux kernel, the following vulnerability has been resolved: sunrpc: harden rq_procinfo lifecycle to prevent double-free The svc_release_rqst() function executes the callback inside rqstp->rq_procinfo->pc_release. However, if a worker thread begins processing a new request and encounters an early error path (e.g., unsupported protocol, short frame, or bad auth) before a valid rq_procinfo is installed, a stale release hook can be re-triggered against reused state from the previous RPC, resulting in a double-free or use-after-free vulnerability. Harden the lifecycle of rq_procinfo by: 1. Ensuring svc_release_rqst() always clears rq_procinfo after the optional pc_release() call, regardless of whether the hook exists. 2. Explicitly clearing rq_procinfo at request entry in svc_process() before any early decode or drop paths. 3. Ensuring svc_process_bc() does the same at backchannel entry. This guarantees that error flows will not encounter a non-NULL stale rq_procinfo pointer when there is nothing to release.