tty: tty_open can return to userspace holding tty_mutex

[Eric Paris <eparis@redhat.com>]

__tty_open could return (to userspace) holding the tty_mutex thanks to a
regression introduced by 4a2b5fddd53b80efcb3266ee36e23b8de28e761a.  This was
found by bisecting an fsfuzzer problem.  Admittedly I have no idea how it
managed to tickle this 100% reliably, but it is clearly a regression and
when hit leaves the box in a completely unusable state.  This patch lets
the fsfuzzer test complete every time.
    
Signed-off-by: Eric Paris <eparis@redhat.com>

---
 drivers/char/tty_io.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index d33e5ab..bc84e12 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -1817,8 +1817,10 @@ got_driver:
 		/* check whether we're reopening an existing tty */
 		tty = tty_driver_lookup_tty(driver, inode, index);
 
-		if (IS_ERR(tty))
+		if (IS_ERR(tty)) {
+			mutex_unlock(&tty_mutex);
 			return PTR_ERR(tty);
+		}
 	}
 
 	if (tty) {