tty: Fix unsafe vt paste_selection()

Convert the tty_buffer_flush() exclusion mechanism to a
public interface - tty_buffer_lock/unlock_exclusive() - and use
the interface to safely write the paste selection to the line
discipline.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/include/linux/tty.h b/include/linux/tty.h
index 991575f..7a9a3b0 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -67,8 +67,8 @@
 struct tty_bufhead {
 	struct tty_buffer *head;	/* Queue head */
 	struct work_struct work;
-	struct mutex flush_mutex;
-	unsigned int flushpending:1;
+	struct mutex	   lock;
+	atomic_t	   priority;
 	struct tty_buffer sentinel;
 	struct llist_head free;		/* Free queue head */
 	atomic_t	   memory_used; /* In-use buffers excluding free list */