tty: replace BKL with a new tty_lock

As a preparation for replacing the big kernel lock
in the TTY layer, wrap all the callers in new
macros tty_lock, tty_lock_nested and tty_unlock.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/drivers/char/pty.c b/drivers/char/pty.c
index b640ef2..622e21c 100644
--- a/drivers/char/pty.c
+++ b/drivers/char/pty.c
@@ -692,9 +692,9 @@
 {
 	int ret;
 
-	lock_kernel();
+	tty_lock();
 	ret = __ptmx_open(inode, filp);
-	unlock_kernel();
+	tty_unlock();
 	return ret;
 }