NFSD: Prevent client use-after-free during delegation revoke

In the Linux kernel, the following vulnerability has been resolved: NFSD: Prevent client use-after-free during delegation revoke A delegation stateid holds only a bare pointer to its owning nfs4_client and does not keep it alive. The client survives its stateids only because __destroy_client() drains cl_delegations and cl_revoked before free_client() runs. nfs4_laundromat() breaks that invariant: it unhashes an expired delegation from cl_delegations, drops deleg_lock, then revoke_delegation() relinks it onto cl_revoked under cl_lock. In that window the delegation is on neither list, so client_has_state() can report no remaining state. Every teardown path first requires cl_rpc_users to be zero, but the laundromat holds no such reference. A client whose recalled delegation has just timed out can therefore reach free_client() while revoke_delegation() is still about to dereference cl_lock, a use-after-free. Pin the client with cl_rpc_users across the revoke so teardown blocks until it completes, then reap the delegation from cl_revoked. A client already expiring reaps its own, so skip it and leave the delegation on del_recall_lru.

Where it's fixed