media: staging: lirc_zilog: incorrect reference counting

[not upstream as the driver is deleted in 4.16 - gregkh]

Whenever poll is called, the reference count is increased but never
decreased. This means that on rmmod, the lirc_thread is not stopped,
and will trample over freed memory.

Zilog/Hauppauge IR driver unloaded
BUG: unable to handle kernel paging request at ffffffffc17ba640
Oops: 0010 [#1] SMP
CPU: 1 PID: 667 Comm: zilog-rx-i2c-1 Tainted: P         C OE   4.13.16-302.fc27.x86_64 #1
Hardware name: Gigabyte Technology Co., Ltd. GA-MA790FXT-UD5P/GA-MA790FXT-UD5P, BIOS F6 08/06/2009
task: ffff964eb452ca00 task.stack: ffffb254414dc000
RIP: 0010:0xffffffffc17ba640
RSP: 0018:ffffb254414dfe78 EFLAGS: 00010286
RAX: 0000000000000000 RBX: ffff964ec1b35890 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000246 RDI: 0000000000000246
RBP: ffffb254414dff00 R08: 000000000000036e R09: ffff964ecfc8dfd0
R10: ffffb254414dfe78 R11: 00000000000f4240 R12: ffff964ec2bf28a0
R13: ffff964ec1b358a8 R14: ffff964ec1b358d0 R15: ffff964ec1b35800
FS:  0000000000000000(0000) GS:ffff964ecfc80000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffc17ba640 CR3: 000000023058c000 CR4: 00000000000006e0
Call Trace:
 kthread+0x125/0x140
 ? kthread_park+0x60/0x60
 ? do_syscall_64+0x67/0x140
 ret_from_fork+0x25/0x30
Code:  Bad RIP value.
RIP: 0xffffffffc17ba640 RSP: ffffb254414dfe78
CR2: ffffffffc17ba640

Note that zilog-rx-i2c-1 should have exited by now, but hasn't due to
the missing put in poll().

This code has been replaced completely in kernel v4.16 by a new driver,
see commit acaa34bf ("media: rc: implement zilog transmitter"), and
commit f95367a7 ("media: staging: remove lirc_zilog driver").

Cc: stable@vger.kernel.org # v4.15- (all up to and including v4.15)
Reported-by: Warren Sturm <warren.sturm@gmail.com>
Tested-by: Warren Sturm <warren.sturm@gmail.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/media/lirc/lirc_zilog.c

@@ -1228,6 +1228,7 @@ static unsigned int poll(struct file *filep, poll_table *wait)
 
 	dev_dbg(ir->l.dev, "%s result = %s\n", __func__,
 		ret ? "POLLIN|POLLRDNORM" : "none");
+	put_ir_rx(rx, false);
 	return ret;
 }