NFSv4.1/pNFS: Fix borken function _same_data_server_addrs_locked()

- Switch back to using list_for_each_entry(). Fixes an incorrect test
  for list NULL termination.
- Do not assume that lists are sorted.
- Finally, consider an existing entry to match if it consists of a subset
  of the addresses in the new entry.

Cc: stable@vger.kernel.org # 4.0+
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/pnfs_nfs.c

@@ -359,26 +359,31 @@ same_sockaddr(struct sockaddr *addr1, struct sockaddr *addr2)
 	return false;
 }
 
+/*
+ * Checks if 'dsaddrs1' contains a subset of 'dsaddrs2'. If it does,
+ * declare a match.
+ */
 static bool
 _same_data_server_addrs_locked(const struct list_head *dsaddrs1,
 			       const struct list_head *dsaddrs2)
 {
 	struct nfs4_pnfs_ds_addr *da1, *da2;
-
-	/* step through both lists, comparing as we go */
-	for (da1 = list_first_entry(dsaddrs1, typeof(*da1), da_node),
-	     da2 = list_first_entry(dsaddrs2, typeof(*da2), da_node);
-	     da1 != NULL && da2 != NULL;
-	     da1 = list_entry(da1->da_node.next, typeof(*da1), da_node),
-	     da2 = list_entry(da2->da_node.next, typeof(*da2), da_node)) {
-		if (!same_sockaddr((struct sockaddr *)&da1->da_addr,
-				   (struct sockaddr *)&da2->da_addr))
-			return false;
+	struct sockaddr *sa1, *sa2;
+	bool match = false;
+
+	list_for_each_entry(da1, dsaddrs1, da_node) {
+		sa1 = (struct sockaddr *)&da1->da_addr;
+		match = false;
+		list_for_each_entry(da2, dsaddrs2, da_node) {
+			sa2 = (struct sockaddr *)&da2->da_addr;
+			match = same_sockaddr(sa1, sa2);
+			if (match)
+				break;
+		}
+		if (!match)
+			break;
 	}
-	if (da1 == NULL && da2 == NULL)
-		return true;
-
-	return false;
+	return match;
 }
 
 /*