bonding: restore bond's IFF_SLAVE flag if a non-eth dev enslave fails

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

bonding: restore bond's IFF_SLAVE flag if a non-eth dev enslave fails

syzbot reported a warning[1] where the bond device itself is a slave and
we try to enslave a non-ethernet device as the first slave which fails
but then in the error path when ether_setup() restores the bond device
it also clears all flags. In my previous fix[2] I restored the
IFF_MASTER flag, but I didn't consider the case that the bond device
itself might also be a slave with IFF_SLAVE set, so we need to restore
that flag as well. Use the bond_ether_setup helper which does the right
thing and restores the bond's flags properly.

Steps to reproduce using a nlmon dev:
$ ip l add nlmon0 type nlmon
$ ip l add bond1 type bond
$ ip l add bond2 type bond
$ ip l set bond1 master bond2
$ ip l set dev nlmon0 master bond1
$ ip -d l sh dev bond1
22: bond1: <BROADCAST,MULTICAST,MASTER> mtu 1500 qdisc noqueue master bond2 state DOWN mode DEFAULT group default qlen 1000
(now bond1's IFF_SLAVE flag is gone and we'll hit a warning[3] if we
try to delete it)

[1] https://syzkaller.appspot.com/bug?id=391c7b1f6522182899efba27d891f1743e8eb3ef
[2] commit 7d5cd2ce5292 ("bonding: correctly handle bonding type change on enslave failure")
[3] example warning:
[ 27.008664] bond1: (slave nlmon0): The slave device specified does not support setting the MAC address
[ 27.008692] bond1: (slave nlmon0): Error -95 calling set_mac_address
[ 32.464639] bond1 (unregistering): Released all slaves
[ 32.464685] ------------[ cut here ]------------
[ 32.464686] WARNING: CPU: 1 PID: 2004 at net/core/dev.c:10829 unregister_netdevice_many+0x72a/0x780
[ 32.464694] Modules linked in: br_netfilter bridge bonding virtio_net
[ 32.464699] CPU: 1 PID: 2004 Comm: ip Kdump: loaded Not tainted 5.18.0-rc3+ #47
[ 32.464703] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.1-2.fc37 04/01/2014
[ 32.464704] RIP: 0010:unregister_netdevice_many+0x72a/0x780
[ 32.464707] Code: 99 fd ff ff ba 90 1a 00 00 48 c7 c6 f4 02 66 96 48 c7 c7 20 4d 35 96 c6 05 fa c7 2b 02 01 e8 be 6f 4a 00 0f 0b e9 73 fd ff ff <0f> 0b e9 5f fd ff ff 80 3d e3 c7 2b 02 00 0f 85 3b fd ff ff ba 59
[ 32.464710] RSP: 0018:ffffa006422d7820 EFLAGS: 00010206
[ 32.464712] RAX: ffff8f6e077140a0 RBX: ffffa006422d7888 RCX: 0000000000000000
[ 32.464714] RDX: ffff8f6e12edbe58 RSI: 0000000000000296 RDI: ffffffff96d4a520
[ 32.464716] RBP: ffff8f6e07714000 R08: ffffffff96d63600 R09: ffffa006422d7728
[ 32.464717] R10: 0000000000000ec0 R11: ffffffff9698c988 R12: ffff8f6e12edb140
[ 32.464719] R13: dead000000000122 R14: dead000000000100 R15: ffff8f6e12edb140
[ 32.464723] FS: 00007f297c2f1740(0000) GS:ffff8f6e5d900000(0000) knlGS:0000000000000000
[ 32.464725] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 32.464726] CR2: 00007f297bf1c800 CR3: 00000000115e8000 CR4: 0000000000350ee0
[ 32.464730] Call Trace:
[ 32.464763] <TASK>
[ 32.464767] rtnl_dellink+0x13e/0x380
[ 32.464776] ? cred_has_capability.isra.0+0x68/0x100
[ 32.464780] ? __rtnl_unlock+0x33/0x60
[ 32.464783] ? bpf_lsm_capset+0x10/0x10
[ 32.464786] ? security_capable+0x36/0x50
[ 32.464790] rtnetlink_rcv_msg+0x14e/0x3b0
[ 32.464792] ? _copy_to_iter+0xb1/0x790
[ 32.464796] ? post_alloc_hook+0xa0/0x160
[ 32.464799] ? rtnl_calcit.isra.0+0x110/0x110
[ 32.464802] netlink_rcv_skb+0x50/0xf0
[ 32.464806] netlink_unicast+0x216/0x340
[ 32.464809] netlink_sendmsg+0x23f/0x480
[ 32.464812] sock_sendmsg+0x5e/0x60
[ 32.464815] ____sys_sendmsg+0x22c/0x270
[ 32.464818] ? import_iovec+0x17/0x20
[ 32.464821] ? sendmsg_copy_msghdr+0x59/0x90
[ 32.464823] ? do_set_pte+0xa0/0xe0
[ 32.464828] ___sys_sendmsg+0x81/0xc0
[ 32.464832] ? mod_objcg_state+0xc6/0x300
[ 32.464835] ? refill_obj_stock+0xa9/0x160
[ 32.464838] ? memcg_slab_free_hook+0x1a5/0x1f0
[ 32.464842] __sys_sendm
---truncated---