kill unused invalidate_inode_pages helper

No one is calling this anymore as everyone has switched to
invalidate_mapping_pages long time ago.  Also update a few
references to it in comments.  nfs has two more, but I can't
easily figure what they are actually referring to, so I left
them as-is.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/drivers/usb/gadget/file_storage.c b/drivers/usb/gadget/file_storage.c
index 29dfb02..7dcdbda 100644
--- a/drivers/usb/gadget/file_storage.c
+++ b/drivers/usb/gadget/file_storage.c
@@ -1448,7 +1448,7 @@
 	unsigned long	rc;
 
 	rc = invalidate_mapping_pages(inode->i_mapping, 0, -1);
-	VLDBG(curlun, "invalidate_inode_pages -> %ld\n", rc);
+	VLDBG(curlun, "invalidate_mapping_pages -> %ld\n", rc);
 }
 
 static int do_verify(struct fsg_dev *fsg)