nfs4: take a reference on the nfs_client when running FREE_STATEID

During umount, the session slot tables are freed.  If there are
outstanding FREE_STATEID tasks, a use-after-free and slab corruption can
occur when rpc_exit_task calls rpc_call_done -> nfs41_sequence_done ->
nfs4_sequence_process/nfs41_sequence_free_slot.

Prevent that from happening by taking a reference on the nfs_client in
nfs41_free_stateid and putting it in nfs41_free_stateid_release.
Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/nfs4proc.c

@@ -10115,6 +10115,10 @@ static void nfs41_free_stateid_done(struct rpc_task *task, void *calldata)
 
 static void nfs41_free_stateid_release(void *calldata)
 {
+	struct nfs_free_stateid_data *data = calldata;
+	struct nfs_client *clp = data->server->nfs_client;
+
+	nfs_put_client(clp);
 	kfree(calldata);
 }
 
@@ -10151,6 +10155,10 @@ static int nfs41_free_stateid(struct nfs_server *server,
 	};
 	struct nfs_free_stateid_data *data;
 	struct rpc_task *task;
+	struct nfs_client *clp = server->nfs_client;
+
+	if (!refcount_inc_not_zero(&clp->cl_count))
+		return -EIO;
 
 	nfs4_state_protect(server->nfs_client, NFS_SP4_MACH_CRED_STATEID,
 		&task_setup.rpc_client, &msg);