SUNRPC: create API for getting remote peer address

Provide an API for retrieving the remote peer address without allowing
direct access to the rpc_xprt struct.

Test-plan:
Compile kernel with CONFIG_NFS enabled.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

include/linux/sunrpc/clnt.h

@@ -123,6 +123,7 @@ void		rpc_setbufsize(struct rpc_clnt *, unsigned int, unsigned int);
 size_t		rpc_max_payload(struct rpc_clnt *);
 void		rpc_force_rebind(struct rpc_clnt *);
 int		rpc_ping(struct rpc_clnt *clnt, int flags);
+size_t		rpc_peeraddr(struct rpc_clnt *, struct sockaddr *, size_t);
 
 /*
  * Helper function for NFSroot support

net/sunrpc/clnt.c

@@ -533,6 +533,27 @@ rpc_call_setup(struct rpc_task *task, struct rpc_message *msg, int flags)
 		task->tk_action = rpc_exit_task;
 }
 
+/**
+ * rpc_peeraddr - extract remote peer address from clnt's xprt
+ * @clnt: RPC client structure
+ * @buf: target buffer
+ * @size: length of target buffer
+ *
+ * Returns the number of bytes that are actually in the stored address.
+ */
+size_t rpc_peeraddr(struct rpc_clnt *clnt, struct sockaddr *buf, size_t bufsize)
+{
+	size_t bytes;
+	struct rpc_xprt *xprt = clnt->cl_xprt;
+
+	bytes = sizeof(xprt->addr);
+	if (bytes > bufsize)
+		bytes = bufsize;
+	memcpy(buf, &clnt->cl_xprt->addr, bytes);
+	return sizeof(xprt->addr);
+}
+EXPORT_SYMBOL(rpc_peeraddr);
+
 void
 rpc_setbufsize(struct rpc_clnt *clnt, unsigned int sndsize, unsigned int rcvsize)
 {