net: core: don't call SIOCBRADD/DELIF for non-bridge devices

Commit ad2f99ae ("net: bridge: move bridge ioctls out of .ndo_do_ioctl")
changed SIOCBRADD/DELIF to use bridge's ioctl hook (br_ioctl_hook)
without checking if the target netdevice is actually a bridge which can
cause crashes and generally interpreting other devices' private pointers
as net_bridge pointers.

Crash example (lo - loopback):
$ brctl addif lo ens16
 BUG: kernel NULL pointer dereference, address: 000000000000059898
 #PF: supervisor read access in kernel modede
 #PF: error_code(0x0000) - not-present pagege
 PGD 0 P4D 0 ^Ac
 Oops: 0000 [#1] SMP NOPTI
 CPU: 2 PID: 1376 Comm: brctl Kdump: loaded Tainted: G        W         5.14.0-rc3+ #405
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-4.fc34 04/01/2014
 RIP: 0010:add_del_if+0x1f/0x7c [bridge]
 Code: 80 bf 1b a0 41 5c e9 c0 3c 03 e1 0f 1f 44 00 00 41 55 41 54 41 89 f4 be 0c 00 00 00 55 48 89 fd 53 48 8b 87 88 00 00 00 89 d3 <4c> 8b a8 98 05 00 00 49 8b bd d0 00 00 00 e8 17 d7 f3 e0 84 c0 74
 RSP: 0018:ffff888109d97cb0 EFLAGS: 00010202^Ac
 RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
 RDX: 0000000000000000 RSI: 000000000000000c RDI: ffff888101239bc0
 RBP: ffff888101239bc0 R08: 0000000000000001 R09: 0000000000000000
 R10: ffff888109d97cd8 R11: 00000000000000a3 R12: 0000000000000012
 R13: 0000000000000000 R14: ffff888101239bc0 R15: ffff888109d97e10
 FS:  00007fc1e365b540(0000) GS:ffff88822be80000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000000000598 CR3: 0000000106506000 CR4: 00000000000006e0
 Call Trace:
  br_ioctl_stub+0x7c/0x441 [bridge]
  br_ioctl_call+0x6d/0x8a
  dev_ifsioc+0x325/0x4e8
  dev_ioctl+0x46b/0x4e1
  sock_do_ioctl+0x7b/0xad
  sock_ioctl+0x2de/0x2f2
  vfs_ioctl+0x1e/0x2b
  __do_sys_ioctl+0x63/0x86
  do_syscall_64+0xcb/0xf2
  entry_SYSCALL_64_after_hwframe+0x44/0xae
 RIP: 0033:0x7fc1e3589427
 Code: 00 00 90 48 8b 05 69 aa 0c 00 64 c7 00 26 00 00 00 48 c7 c0 ff ff ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 b8 10 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 39 aa 0c 00 f7 d8 64 89 01 48
 RSP: 002b:00007ffc8d501d38 EFLAGS: 00000202 ORIG_RAX: 000000000000001010
 RAX: ffffffffffffffda RBX: 0000000000000012 RCX: 00007fc1e3589427
 RDX: 00007ffc8d501d60 RSI: 00000000000089a3 RDI: 0000000000000003
 RBP: 00007ffc8d501d60 R08: 0000000000000000 R09: fefefeff77686d74
 R10: fffffffffffff8f9 R11: 0000000000000202 R12: 00007ffc8d502e06
 R13: 00007ffc8d502e06 R14: 0000000000000000 R15: 0000000000000000
 Modules linked in: bridge stp llc bonding ipv6 virtio_net [last unloaded: llc]^Ac
 CR2: 0000000000000598

Reported-by:  <syzbot+79f4a8692e267bdb7227@syzkaller.appspotmail.com>
Fixes: ad2f99ae ("net: bridge: move bridge ioctls out of .ndo_do_ioctl")
Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>