tty: Don't call panic() at tty_ldisc_init()

commit 903f9db10f18f735e62ba447147b6c434b6af003 upstream.

syzbot is reporting kernel panic [1] triggered by memory allocation failure
at tty_ldisc_get() from tty_ldisc_init(). But since both tty_ldisc_get()
and caller of tty_ldisc_init() can cleanly handle errors, tty_ldisc_init()
does not need to call panic() when tty_ldisc_get() failed.

[1] https://syzkaller.appspot.com/bug?id=883431818e036ae6a9981156a64b821110f39187

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: syzbot <syzkaller@googlegroups.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index 3a9e2a2..96f0d10 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -753,12 +753,13 @@
  *	the tty structure is not completely set up when this call is made.
  */
 
-void tty_ldisc_init(struct tty_struct *tty)
+int tty_ldisc_init(struct tty_struct *tty)
 {
 	struct tty_ldisc *ld = tty_ldisc_get(tty, N_TTY);
 	if (IS_ERR(ld))
-		panic("n_tty: init_tty");
+		return PTR_ERR(ld);
 	tty->ldisc = ld;
+	return 0;
 }
 
 /**