vxlan: do not destroy fdb if register_netdevice() is failed

vxlan: do not destroy fdb if register_netdevice() is failed

__vxlan_dev_create() destroys FDB using specific pointer which indicates
a fdb when error occurs.
But that pointer should not be used when register_netdevice() fails because
register_netdevice() internally destroys fdb when error occurs.

This patch makes vxlan_fdb_create() to do not link fdb entry to vxlan dev
internally.
Instead, a new function vxlan_fdb_insert() is added to link fdb to vxlan
dev.

vxlan_fdb_insert() is called after calling register_netdevice().
This routine can avoid situation that ->ndo_uninit() destroys fdb entry
in error path of register_netdevice().
Hence, error path of __vxlan_dev_create() routine can have an opportunity
to destroy default fdb entry by hand.

Test command
    ip link add bonding_masters type vxlan id 0 group 239.1.1.1 \
	    dev enp0s9 dstport 4789

Splat looks like:
[  213.392816] kasan: GPF could be caused by NULL-ptr deref or user memory access
[  213.401257] general protection fault: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN PTI
[  213.402178] CPU: 0 PID: 1414 Comm: ip Not tainted 5.2.0-rc5+ #256
[  213.402178] RIP: 0010:vxlan_fdb_destroy+0x120/0x220 [vxlan]
[  213.402178] Code: df 48 8b 2b 48 89 fa 48 c1 ea 03 80 3c 02 00 0f 85 06 01 00 00 4c 8b 63 08 48 b8 00 00 00 00 00 fc d
[  213.402178] RSP: 0018:ffff88810cb9f0a0 EFLAGS: 00010202
[  213.402178] RAX: dffffc0000000000 RBX: ffff888101d4a8c8 RCX: 0000000000000000
[  213.402178] RDX: 1bd5a00000000040 RSI: ffff888101d4a8c8 RDI: ffff888101d4a8d0
[  213.402178] RBP: 0000000000000000 R08: fffffbfff22b72d9 R09: 0000000000000000
[  213.402178] R10: 00000000ffffffef R11: 0000000000000000 R12: dead000000000200
[  213.402178] R13: ffff88810cb9f1f8 R14: ffff88810efccda0 R15: ffff88810efccda0
[  213.402178] FS:  00007f7f6621a0c0(0000) GS:ffff88811b000000(0000) knlGS:0000000000000000
[  213.402178] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  213.402178] CR2: 000055746f0807d0 CR3: 00000001123e0000 CR4: 00000000001006f0
[  213.402178] Call Trace:
[  213.402178]  __vxlan_dev_create+0x3a9/0x7d0 [vxlan]
[  213.402178]  ? vxlan_changelink+0x740/0x740 [vxlan]
[  213.402178]  ? rcu_read_unlock+0x60/0x60 [vxlan]
[  213.402178]  ? __kasan_kmalloc.constprop.3+0xa0/0xd0
[  213.402178]  vxlan_newlink+0x8d/0xc0 [vxlan]
[  213.402178]  ? __vxlan_dev_create+0x7d0/0x7d0 [vxlan]
[  213.554119]  ? __netlink_ns_capable+0xc3/0xf0
[  213.554119]  __rtnl_newlink+0xb75/0x1180
[  213.554119]  ? rtnl_link_unregister+0x230/0x230
[ ... ]

Fixes: 0241b836732f ("vxlan: fix default fdb entry netlink notify ordering during netdev create")
Suggested-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>