SUNRPC: Fix infinite looping in rpc_clnt_iterate_for_each_xprt

If there were less than 2 entries in the multipath list, then
xprt_iter_next_entry_multiple() would never advance beyond the
first entry, which is correct for round robin behaviour, but not
for the list iteration.

The end result would be infinite looping in rpc_clnt_iterate_for_each_xprt()
as we would never see the xprt == NULL condition fulfilled.

Reported-by: Oleg Drokin <green@linuxhacker.ru>
Fixes: 80b14d5e61ca ("SUNRPC: Add a structure to track multiple transports")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

net/sunrpc/xprtmultipath.c

@@ -271,14 +271,12 @@ struct rpc_xprt *xprt_iter_next_entry_multiple(struct rpc_xprt_iter *xpi,
 		xprt_switch_find_xprt_t find_next)
 {
 	struct rpc_xprt_switch *xps = rcu_dereference(xpi->xpi_xpswitch);
-	struct list_head *head;
 
 	if (xps == NULL)
 		return NULL;
-	head = &xps->xps_xprt_list;
-	if (xps->xps_nxprts < 2)
-		return xprt_switch_find_first_entry(head);
-	return xprt_switch_set_next_cursor(head, &xpi->xpi_cursor, find_next);
+	return xprt_switch_set_next_cursor(&xps->xps_xprt_list,
+			&xpi->xpi_cursor,
+			find_next);
 }
 
 static