[PATCH] tty/n_hdlc: fix __might_sleep warning

[Paul Fulghum <paulkf@microgate.com>]

Fix __might_sleep warning in tty/n_hdlc.c read due to copy_to_user call while current is TASK_INTERRUPTIBLE.
This is a false positive since the code path does not depend on current state remaining TASK_INTERRUPTIBLE.
The loop breaks out and sets TASK_RUNNING after calling copy_to_user. 
This patch supresses the warning by setting TASK_RUNNING before calling copy_to_user.

[1] https://syzkaller.appspot.com/bug?id=17d5de7f1fcab794cb8c40032f893f52de899324

Signed-off-by: Paul Fulghum <paulkf@microgate.com>
Reported-by: syzbot <syzbot+c244af085a0159d22879@syzkaller.appspotmail.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
—
--- a/drivers/tty/n_hdlc.c	2018-12-23 15:55:59.000000000 -0800
+++ b/drivers/tty/n_hdlc.c	2019-01-01 11:44:47.148153954 -0800
@@ -597,6 +597,7 @@ static ssize_t n_hdlc_tty_read(struct tt
 				/* too large for caller's buffer */
 				ret = -EOVERFLOW;
 			} else {
+				__set_current_state(TASK_RUNNING);
 				if (copy_to_user(buf, rbuf->buf, rbuf->count))
 					ret = -EFAULT;
 				else