xfrm interface: fix memory leak on creation

The following commands produce a backtrace and return an error but the xfrm
interface is created (in the wrong netns):
$ ip netns add foo
$ ip netns add bar
$ ip -n foo netns set bar 0
$ ip -n foo link add xfrmi0 link-netnsid 0 type xfrm dev lo if_id 23
RTNETLINK answers: Invalid argument
$ ip -n bar link ls xfrmi0
2: xfrmi0@lo: <NOARP,M-DOWN> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/none 00:00:00:00:00:00 brd 00:00:00:00:00:00

Here is the backtrace:
[   79.879174] WARNING: CPU: 0 PID: 1178 at net/core/dev.c:8172 rollback_registered_many+0x86/0x3c1
[   79.880260] Modules linked in: xfrm_interface nfsv3 nfs_acl auth_rpcgss nfsv4 nfs lockd grace sunrpc fscache button parport_pc parport serio_raw evdev pcspkr loop ext4 crc16 mbcache jbd2 crc32c_generic ide_cd_mod ide_gd_mod cdrom ata_$
eneric ata_piix libata scsi_mod 8139too piix psmouse i2c_piix4 ide_core 8139cp mii i2c_core floppy
[   79.883698] CPU: 0 PID: 1178 Comm: ip Not tainted 5.2.0-rc6+ #106
[   79.884462] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
[   79.885447] RIP: 0010:rollback_registered_many+0x86/0x3c1
[   79.886120] Code: 01 e8 d7 7d c6 ff 0f 0b 48 8b 45 00 4c 8b 20 48 8d 58 90 49 83 ec 70 48 8d 7b 70 48 39 ef 74 44 8a 83 d0 04 00 00 84 c0 75 1f <0f> 0b e8 61 cd ff ff 48 b8 00 01 00 00 00 00 ad de 48 89 43 70 66
[   79.888667] RSP: 0018:ffffc900015ab740 EFLAGS: 00010246
[   79.889339] RAX: ffff8882353e5700 RBX: ffff8882353e56a0 RCX: ffff8882353e5710
[   79.890174] RDX: ffffc900015ab7e0 RSI: ffffc900015ab7e0 RDI: ffff8882353e5710
[   79.891029] RBP: ffffc900015ab7e0 R08: ffffc900015ab7e0 R09: ffffc900015ab7e0
[   79.891866] R10: ffffc900015ab7a0 R11: ffffffff82233fec R12: ffffc900015ab770
[   79.892728] R13: ffffffff81eb7ec0 R14: ffff88822ed6cf00 R15: 00000000ffffffea
[   79.893557] FS:  00007ff350f31740(0000) GS:ffff888237a00000(0000) knlGS:0000000000000000
[   79.894581] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   79.895317] CR2: 00000000006c8580 CR3: 000000022c272000 CR4: 00000000000006f0
[   79.896137] Call Trace:
[   79.896464]  unregister_netdevice_many+0x12/0x6c
[   79.896998]  __rtnl_newlink+0x6e2/0x73b
[   79.897446]  ? __kmalloc_node_track_caller+0x15e/0x185
[   79.898039]  ? pskb_expand_head+0x5f/0x1fe
[   79.898556]  ? stack_access_ok+0xd/0x2c
[   79.899009]  ? deref_stack_reg+0x12/0x20
[   79.899462]  ? stack_access_ok+0xd/0x2c
[   79.899927]  ? stack_access_ok+0xd/0x2c
[   79.900404]  ? __module_text_address+0x9/0x4f
[   79.900910]  ? is_bpf_text_address+0x5/0xc
[   79.901390]  ? kernel_text_address+0x67/0x7b
[   79.901884]  ? __kernel_text_address+0x1a/0x25
[   79.902397]  ? unwind_get_return_address+0x12/0x23
[   79.903122]  ? __cmpxchg_double_slab.isra.37+0x46/0x77
[   79.903772]  rtnl_newlink+0x43/0x56
[   79.904217]  rtnetlink_rcv_msg+0x200/0x24c

In fact, each time a xfrm interface was created, a netdev was allocated
by __rtnl_newlink()/rtnl_create_link() and then another one by
xfrmi_newlink()/xfrmi_create(). Only the second one was registered, it's
why the previous commands produce a backtrace: dev_change_net_namespace()
was called on a netdev with reg_state set to NETREG_UNINITIALIZED (the
first one).

CC: Lorenzo Colitti <lorenzo@google.com>
CC: Benedict Wong <benedictwong@google.com>
CC: Steffen Klassert <steffen.klassert@secunet.com>
CC: Shannon Nelson <shannon.nelson@oracle.com>
CC: Antony Antony <antony@phenome.org>
CC: Eyal Birger <eyal.birger@gmail.com>
Fixes: f203b76d ("xfrm: Add virtual xfrm interfaces")
Reported-by: Julien Floret <julien.floret@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>