tty: release BTM while sleeping in block_til_ready

Most tty drivers may block while opening a device.
Since this possibly depends on another thread
closing it first and both threads may need the BTM,
we need to release it here.

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/serial/68328serial.c b/drivers/serial/68328serial.c
index 9330edb..d8204f4 100644
--- a/drivers/serial/68328serial.c
+++ b/drivers/serial/68328serial.c
@@ -1235,7 +1235,9 @@
 			retval = -ERESTARTSYS;
 			break;
 		}
+		tty_unlock();
 		schedule();
+		tty_lock();
 	}
 	current->state = TASK_RUNNING;
 	remove_wait_queue(&info->open_wait, &wait);