net: hamradio: fix memory leak in mkiss_close

In the Linux kernel, the following vulnerability has been resolved:

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