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/vc_screen.c b/drivers/char/vc_screen.c
index c1791a6..bcce46c 100644
--- a/drivers/char/vc_screen.c
+++ b/drivers/char/vc_screen.c
@@ -463,10 +463,10 @@
 	unsigned int currcons = iminor(inode) & 127;
 	int ret = 0;
 	
-	lock_kernel();
+	tty_lock();
 	if(currcons && !vc_cons_allocated(currcons-1))
 		ret = -ENXIO;
-	unlock_kernel();
+	tty_unlock();
 	return ret;
 }