tty: Rename tty buffer memory_used field

Trim up the memory_used field name to mem_used.

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 2225745..38fcc05 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -60,7 +60,7 @@
 	atomic_t	   priority;
 	struct tty_buffer sentinel;
 	struct llist_head free;		/* Free queue head */
-	atomic_t	   memory_used; /* In-use buffers excluding free list */
+	atomic_t	   mem_used;    /* In-use buffers excluding free list */
 	int		   mem_limit;
 	struct tty_buffer *tail;	/* Active buffer */
 };