Use derived-mode-p consistently in info.el

* lisp/info.el (Info-hide-note-references, info-display-manual)
(info--manual-names): Use derived-mode-p.  (Bug#27583)

lisp/info.el

@@ -318,7 +318,7 @@ want to set `Info-refill-paragraphs'."
 	 (set sym val)
 	 (dolist (buffer (buffer-list))
 	   (with-current-buffer buffer
-	     (when (eq major-mode 'Info-mode)
+             (when (derived-mode-p 'Info-mode)
 	       (revert-buffer t t)))))
   :group 'info)
 
@@ -5332,7 +5332,7 @@ completion alternatives to currently visited manuals."
 	found)
     (dolist (buffer blist)
       (with-current-buffer buffer
-	(when (and (eq major-mode 'Info-mode)
+        (when (and (derived-mode-p 'Info-mode)
 		   (stringp Info-current-file)
 		   (string-match manual-re Info-current-file))
 	  (setq found buffer
@@ -5347,7 +5347,7 @@ completion alternatives to currently visited manuals."
   (let (names)
     (dolist (buffer (buffer-list))
       (with-current-buffer buffer
-	(and (eq major-mode 'Info-mode)
+        (and (derived-mode-p 'Info-mode)
 	     (stringp Info-current-file)
 	     (not (string= (substring (buffer-name) 0 1) " "))
 	     (push (file-name-sans-extension