vxlan: move debug check after netdev unregister

vxlan: move debug check after netdev unregister

The debug check must be done after unregister_netdevice_many() call --
the hlist_del_rcu() for this is done inside .ndo_stop.

This is the same with commit 0fda7600c2e1 ("geneve: move debug check after
netdev unregister")

Test commands:
    ip netns del A
    ip netns add A
    ip netns add B

    ip netns exec B ip link add vxlan0 type vxlan vni 100 local 10.0.0.1 \
	    remote 10.0.0.2 dstport 4789 srcport 4789 4789
    ip netns exec B ip link set vxlan0 netns A
    ip netns exec A ip link set vxlan0 up
    ip netns del B

Splat looks like:
[   73.176249][    T7] ------------[ cut here ]------------
[   73.178662][    T7] WARNING: CPU: 4 PID: 7 at drivers/net/vxlan.c:4743 vxlan_exit_batch_net+0x52e/0x720 [vxlan]
[   73.182597][    T7] Modules linked in: vxlan openvswitch nsh nf_conncount nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 mlx5_core nfp mlxfw ixgbevf tls sch_fq_codel nf_tables nfnetlink ip_tables x_tables unix
[   73.190113][    T7] CPU: 4 PID: 7 Comm: kworker/u16:0 Not tainted 5.11.0-rc7+ #838
[   73.193037][    T7] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
[   73.196986][    T7] Workqueue: netns cleanup_net
[   73.198946][    T7] RIP: 0010:vxlan_exit_batch_net+0x52e/0x720 [vxlan]
[   73.201509][    T7] Code: 00 01 00 00 0f 84 39 fd ff ff 48 89 ca 48 c1 ea 03 80 3c 1a 00 0f 85 a6 00 00 00 89 c2 48 83 c2 02 49 8b 14 d4 48 85 d2 74 ce <0f> 0b eb ca e8 b9 51 db dd 84 c0 0f 85 4a fe ff ff 48 c7 c2 80 bc
[   73.208813][    T7] RSP: 0018:ffff888100907c10 EFLAGS: 00010286
[   73.211027][    T7] RAX: 000000000000003c RBX: dffffc0000000000 RCX: ffff88800ec411f0
[   73.213702][    T7] RDX: ffff88800a278000 RSI: ffff88800fc78c70 RDI: ffff88800fc78070
[   73.216169][    T7] RBP: ffff88800b5cbdc0 R08: fffffbfff424de61 R09: fffffbfff424de61
[   73.218463][    T7] R10: ffffffffa126f307 R11: fffffbfff424de60 R12: ffff88800ec41000
[   73.220794][    T7] R13: ffff888100907d08 R14: ffff888100907c50 R15: ffff88800fc78c40
[   73.223337][    T7] FS:  0000000000000000(0000) GS:ffff888114800000(0000) knlGS:0000000000000000
[   73.225814][    T7] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   73.227616][    T7] CR2: 0000562b5cb4f4d0 CR3: 0000000105fbe001 CR4: 00000000003706e0
[   73.229700][    T7] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   73.231820][    T7] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[   73.233844][    T7] Call Trace:
[   73.234698][    T7]  ? vxlan_err_lookup+0x3c0/0x3c0 [vxlan]
[   73.235962][    T7]  ? ops_exit_list.isra.11+0x93/0x140
[   73.237134][    T7]  cleanup_net+0x45e/0x8a0
[ ... ]

Fixes: 57b61127ab7d ("vxlan: speedup vxlan tunnels dismantle")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Link: https://lore.kernel.org/r/20210221154552.11749-1-ap420073@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>