virt_wifi: fix refcnt leak in module exit routine

virt_wifi: fix refcnt leak in module exit routine

virt_wifi_newlink() calls netdev_upper_dev_link() and it internally
holds reference count of lower interface.

Current code does not release a reference count of the lower interface
when the lower interface is being deleted.
So, reference count leaks occur.

Test commands:
    ip link add dummy0 type dummy
    ip link add vw1 link dummy0 type virt_wifi
    ip link del dummy0

Splat looks like:
[  133.787526][  T788] WARNING: CPU: 1 PID: 788 at net/core/dev.c:8274 rollback_registered_many+0x835/0xc80
[  133.788355][  T788] Modules linked in: virt_wifi cfg80211 dummy team af_packet sch_fq_codel ip_tables x_tables unix
[  133.789377][  T788] CPU: 1 PID: 788 Comm: ip Not tainted 5.4.0-rc3+ #96
[  133.790069][  T788] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
[  133.791167][  T788] RIP: 0010:rollback_registered_many+0x835/0xc80
[  133.791906][  T788] Code: 00 4d 85 ff 0f 84 b5 fd ff ff ba c0 0c 00 00 48 89 de 4c 89 ff e8 9b 58 04 00 48 89 df e8 30
[  133.794317][  T788] RSP: 0018:ffff88805ba3f338 EFLAGS: 00010202
[  133.795080][  T788] RAX: ffff88805e57e801 RBX: ffff88805ba34000 RCX: ffffffffa9294723
[  133.796045][  T788] RDX: 1ffff1100b746816 RSI: 0000000000000008 RDI: ffffffffabcc4240
[  133.797006][  T788] RBP: ffff88805ba3f4c0 R08: fffffbfff5798849 R09: fffffbfff5798849
[  133.797993][  T788] R10: 0000000000000001 R11: fffffbfff5798848 R12: dffffc0000000000
[  133.802514][  T788] R13: ffff88805ba3f440 R14: ffff88805ba3f400 R15: ffff88805ed622c0
[  133.803237][  T788] FS:  00007f2e9608c0c0(0000) GS:ffff88806cc00000(0000) knlGS:0000000000000000
[  133.804002][  T788] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  133.804664][  T788] CR2: 00007f2e95610603 CR3: 000000005f68c004 CR4: 00000000000606e0
[  133.805363][  T788] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  133.806073][  T788] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  133.806787][  T788] Call Trace:
[  133.807069][  T788]  ? generic_xdp_install+0x310/0x310
[  133.807612][  T788]  ? lock_acquire+0x164/0x3b0
[  133.808077][  T788]  ? is_bpf_text_address+0x5/0xf0
[  133.808640][  T788]  ? deref_stack_reg+0x9c/0xd0
[  133.809138][  T788]  ? __nla_validate_parse+0x98/0x1ab0
[  133.809944][  T788]  unregister_netdevice_many.part.122+0x13/0x1b0
[  133.810599][  T788]  rtnl_delete_link+0xbc/0x100
[  133.811073][  T788]  ? rtnl_af_register+0xc0/0xc0
[  133.811672][  T788]  rtnl_dellink+0x30e/0x8a0
[  133.812205][  T788]  ? is_bpf_text_address+0x5/0xf0
[ ... ]

[  144.110530][  T788] unregister_netdevice: waiting for dummy0 to become free. Usage count = 1

This patch adds notifier routine to delete upper interface before deleting
lower interface.

Fixes: 4a7face1a5ad ("mac80211-next: rtnetlink wifi simulation device")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>