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/char/serial167.c b/drivers/char/serial167.c
index 90b3ec0..f646725 100644
--- a/drivers/char/serial167.c
+++ b/drivers/char/serial167.c
@@ -1786,7 +1786,9 @@
 		       tty->name, info->count);
 		/**/
 #endif
-		    schedule();
+		tty_unlock();
+		schedule();
+		tty_lock();
 	}
 	__set_current_state(TASK_RUNNING);
 	remove_wait_queue(&info->open_wait, &wait);