net: hamradio: fix memory leak in mkiss_close

My local syzbot instance hit memory leak in
mkiss_open()[1]. The problem was in missing
free_netdev() in mkiss_close().

In mkiss_open() netdevice is allocated and then
registered, but in mkiss_close() netdevice was
only unregistered, but not freed.

Fail log:

BUG: memory leak
unreferenced object 0xffff8880281ba000 (size 4096):
  comm "syz-executor.1", pid 11443, jiffies 4295046091 (age 17.660s)
  hex dump (first 32 bytes):
    61 78 30 00 00 00 00 00 00 00 00 00 00 00 00 00  ax0.............
    00 27 fa 2a 80 88 ff ff 00 00 00 00 00 00 00 00  .'.*............
  backtrace:
    [<ffffffff81a27201>] kvmalloc_node+0x61/0xf0
    [<ffffffff8706e7e8>] alloc_netdev_mqs+0x98/0xe80
    [<ffffffff84e64192>] mkiss_open+0xb2/0x6f0 [1]
    [<ffffffff842355db>] tty_ldisc_open+0x9b/0x110
    [<ffffffff84236488>] tty_set_ldisc+0x2e8/0x670
    [<ffffffff8421f7f3>] tty_ioctl+0xda3/0x1440
    [<ffffffff81c9f273>] __x64_sys_ioctl+0x193/0x200
    [<ffffffff8911263a>] do_syscall_64+0x3a/0xb0
    [<ffffffff89200068>] entry_SYSCALL_64_after_hwframe+0x44/0xae

BUG: memory leak
unreferenced object 0xffff8880141a9a00 (size 96):
  comm "syz-executor.1", pid 11443, jiffies 4295046091 (age 17.660s)
  hex dump (first 32 bytes):
    e8 a2 1b 28 80 88 ff ff e8 a2 1b 28 80 88 ff ff  ...(.......(....
    98 92 9c aa b0 40 02 00 00 00 00 00 00 00 00 00  .....@..........
  backtrace:
    [<ffffffff8709f68b>] __hw_addr_create_ex+0x5b/0x310
    [<ffffffff8709fb38>] __hw_addr_add_ex+0x1f8/0x2b0
    [<ffffffff870a0c7b>] dev_addr_init+0x10b/0x1f0
    [<ffffffff8706e88b>] alloc_netdev_mqs+0x13b/0xe80
    [<ffffffff84e64192>] mkiss_open+0xb2/0x6f0 [1]
    [<ffffffff842355db>] tty_ldisc_open+0x9b/0x110
    [<ffffffff84236488>] tty_set_ldisc+0x2e8/0x670
    [<ffffffff8421f7f3>] tty_ioctl+0xda3/0x1440
    [<ffffffff81c9f273>] __x64_sys_ioctl+0x193/0x200
    [<ffffffff8911263a>] do_syscall_64+0x3a/0xb0
    [<ffffffff89200068>] entry_SYSCALL_64_after_hwframe+0x44/0xae

BUG: memory leak
unreferenced object 0xffff8880219bfc00 (size 512):
  comm "syz-executor.1", pid 11443, jiffies 4295046091 (age 17.660s)
  hex dump (first 32 bytes):
    00 a0 1b 28 80 88 ff ff 80 8f b1 8d ff ff ff ff  ...(............
    80 8f b1 8d ff ff ff ff 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<ffffffff81a27201>] kvmalloc_node+0x61/0xf0
    [<ffffffff8706eec7>] alloc_netdev_mqs+0x777/0xe80
    [<ffffffff84e64192>] mkiss_open+0xb2/0x6f0 [1]
    [<ffffffff842355db>] tty_ldisc_open+0x9b/0x110
    [<ffffffff84236488>] tty_set_ldisc+0x2e8/0x670
    [<ffffffff8421f7f3>] tty_ioctl+0xda3/0x1440
    [<ffffffff81c9f273>] __x64_sys_ioctl+0x193/0x200
    [<ffffffff8911263a>] do_syscall_64+0x3a/0xb0
    [<ffffffff89200068>] entry_SYSCALL_64_after_hwframe+0x44/0xae

BUG: memory leak
unreferenced object 0xffff888029b2b200 (size 256):
  comm "syz-executor.1", pid 11443, jiffies 4295046091 (age 17.660s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<ffffffff81a27201>] kvmalloc_node+0x61/0xf0
    [<ffffffff8706f062>] alloc_netdev_mqs+0x912/0xe80
    [<ffffffff84e64192>] mkiss_open+0xb2/0x6f0 [1]
    [<ffffffff842355db>] tty_ldisc_open+0x9b/0x110
    [<ffffffff84236488>] tty_set_ldisc+0x2e8/0x670
    [<ffffffff8421f7f3>] tty_ioctl+0xda3/0x1440
    [<ffffffff81c9f273>] __x64_sys_ioctl+0x193/0x200
    [<ffffffff8911263a>] do_syscall_64+0x3a/0xb0
    [<ffffffff89200068>] entry_SYSCALL_64_after_hwframe+0x44/0xae

Fixes: 815f62bf ("[PATCH] SMP rewrite of mkiss")
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/hamradio/mkiss.c

@@ -799,6 +799,7 @@ static void mkiss_close(struct tty_struct *tty)
 	ax->tty = NULL;
 
 	unregister_netdev(ax->dev);
+	free_netdev(ax->dev);
 }
 
 /* Perform I/O control on an active ax25 channel. */