SUNRPC: Fix suspicious RCU usage

We need to hold the rcu_read_lock() when calling rcu_dereference(),
otherwise we can't guarantee that the object being dereferenced still
exists.

Fixes: 39e5d2df ("SUNRPC search xprt switch for sockaddr")
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/clnt.c

@@ -2753,14 +2753,18 @@ EXPORT_SYMBOL_GPL(rpc_cap_max_reconnect_timeout);
 
 void rpc_clnt_xprt_switch_put(struct rpc_clnt *clnt)
 {
+	rcu_read_lock();
 	xprt_switch_put(rcu_dereference(clnt->cl_xpi.xpi_xpswitch));
+	rcu_read_unlock();
 }
 EXPORT_SYMBOL_GPL(rpc_clnt_xprt_switch_put);
 
 void rpc_clnt_xprt_switch_add_xprt(struct rpc_clnt *clnt, struct rpc_xprt *xprt)
 {
+	rcu_read_lock();
 	rpc_xprt_switch_add_xprt(rcu_dereference(clnt->cl_xpi.xpi_xpswitch),
 				 xprt);
+	rcu_read_unlock();
 }
 EXPORT_SYMBOL_GPL(rpc_clnt_xprt_switch_add_xprt);
 
@@ -2770,9 +2774,8 @@ bool rpc_clnt_xprt_switch_has_addr(struct rpc_clnt *clnt,
 	struct rpc_xprt_switch *xps;
 	bool ret;
 
-	xps = rcu_dereference(clnt->cl_xpi.xpi_xpswitch);
-
 	rcu_read_lock();
+	xps = rcu_dereference(clnt->cl_xpi.xpi_xpswitch);
 	ret = rpc_xprt_switch_has_addr(xps, sap);
 	rcu_read_unlock();
 	return ret;