tty: Make tty_files_lock per-tty

Access to tty->tty_files list is always per-tty, never for all ttys
simultaneously. Replace global tty_files_lock spinlock with per-tty
->files_lock. Initialize when the ->tty_files list is inited, in
alloc_tty_struct().

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 da16b59..780adbf 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -302,6 +302,7 @@
 	struct work_struct hangup_work;
 	void *disc_data;
 	void *driver_data;
+	spinlock_t files_lock;		/* protects tty_files list */
 	struct list_head tty_files;
 
 #define N_TTY_BUF_SIZE 4096
@@ -508,7 +509,6 @@
 		struct tty_struct *tty);
 
 extern struct mutex tty_mutex;
-extern spinlock_t tty_files_lock;
 
 #define tty_is_writelocked(tty)  (mutex_is_locked(&tty->atomic_write_lock))