netlink: do not hard code device address lenth in fdb dumps

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

netlink: do not hard code device address lenth in fdb dumps

syzbot reports that some netdev devices do not have a six bytes
address [1]

Replace ETH_ALEN by dev->addr_len.

[1] (Case of a device where dev->addr_len = 4)

BUG: KMSAN: kernel-infoleak in instrument_copy_to_user include/linux/instrumented.h:114 [inline]
BUG: KMSAN: kernel-infoleak in copyout+0xb8/0x100 lib/iov_iter.c:169
instrument_copy_to_user include/linux/instrumented.h:114 [inline]
copyout+0xb8/0x100 lib/iov_iter.c:169
_copy_to_iter+0x6d8/0x1d00 lib/iov_iter.c:536
copy_to_iter include/linux/uio.h:206 [inline]
simple_copy_to_iter+0x68/0xa0 net/core/datagram.c:513
__skb_datagram_iter+0x123/0xdc0 net/core/datagram.c:419
skb_copy_datagram_iter+0x5c/0x200 net/core/datagram.c:527
skb_copy_datagram_msg include/linux/skbuff.h:3960 [inline]
netlink_recvmsg+0x4ae/0x15a0 net/netlink/af_netlink.c:1970
sock_recvmsg_nosec net/socket.c:1019 [inline]
sock_recvmsg net/socket.c:1040 [inline]
____sys_recvmsg+0x283/0x7f0 net/socket.c:2722
___sys_recvmsg+0x223/0x840 net/socket.c:2764
do_recvmmsg+0x4f9/0xfd0 net/socket.c:2858
__sys_recvmmsg net/socket.c:2937 [inline]
__do_sys_recvmmsg net/socket.c:2960 [inline]
__se_sys_recvmmsg net/socket.c:2953 [inline]
__x64_sys_recvmmsg+0x397/0x490 net/socket.c:2953
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

Uninit was stored to memory at:
__nla_put lib/nlattr.c:1009 [inline]
nla_put+0x1c6/0x230 lib/nlattr.c:1067
nlmsg_populate_fdb_fill+0x2b8/0x600 net/core/rtnetlink.c:4071
nlmsg_populate_fdb net/core/rtnetlink.c:4418 [inline]
ndo_dflt_fdb_dump+0x616/0x840 net/core/rtnetlink.c:4456
rtnl_fdb_dump+0x14ff/0x1fc0 net/core/rtnetlink.c:4629
netlink_dump+0x9d1/0x1310 net/netlink/af_netlink.c:2268
netlink_recvmsg+0xc5c/0x15a0 net/netlink/af_netlink.c:1995
sock_recvmsg_nosec+0x7a/0x120 net/socket.c:1019
____sys_recvmsg+0x664/0x7f0 net/socket.c:2720
___sys_recvmsg+0x223/0x840 net/socket.c:2764
do_recvmmsg+0x4f9/0xfd0 net/socket.c:2858
__sys_recvmmsg net/socket.c:2937 [inline]
__do_sys_recvmmsg net/socket.c:2960 [inline]
__se_sys_recvmmsg net/socket.c:2953 [inline]
__x64_sys_recvmmsg+0x397/0x490 net/socket.c:2953
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

Uninit was created at:
slab_post_alloc_hook+0x12d/0xb60 mm/slab.h:716
slab_alloc_node mm/slub.c:3451 [inline]
__kmem_cache_alloc_node+0x4ff/0x8b0 mm/slub.c:3490
kmalloc_trace+0x51/0x200 mm/slab_common.c:1057
kmalloc include/linux/slab.h:559 [inline]
__hw_addr_create net/core/dev_addr_lists.c:60 [inline]
__hw_addr_add_ex+0x2e5/0x9e0 net/core/dev_addr_lists.c:118
__dev_mc_add net/core/dev_addr_lists.c:867 [inline]
dev_mc_add+0x9a/0x130 net/core/dev_addr_lists.c:885
igmp6_group_added+0x267/0xbc0 net/ipv6/mcast.c:680
ipv6_mc_up+0x296/0x3b0 net/ipv6/mcast.c:2754
ipv6_mc_remap+0x1e/0x30 net/ipv6/mcast.c:2708
addrconf_type_change net/ipv6/addrconf.c:3731 [inline]
addrconf_notify+0x4d3/0x1d90 net/ipv6/addrconf.c:3699
notifier_call_chain kernel/notifier.c:93 [inline]
raw_notifier_call_chain+0xe4/0x430 kernel/notifier.c:461
call_netdevice_notifiers_info net/core/dev.c:1935 [inline]
call_netdevice_notifiers_extack net/core/dev.c:1973 [inline]
call_netdevice_notifiers+0x1ee/0x2d0 net/core/dev.c:1987
bond_enslave+0xccd/0x53f0 drivers/net/bonding/bond_main.c:1906
do_set_master net/core/rtnetlink.c:2626 [inline]
rtnl_newlink_create net/core/rtnetlink.c:3460 [inline]
__rtnl_newlink net/core/rtnetlink.c:3660 [inline]
rtnl_newlink+0x378c/0x40e0 net/core/rtnetlink.c:3673
rtnetlink_rcv_msg+0x16a6/0x1840 net/core/rtnetlink.c:6395
netlink_rcv_skb+0x371/0x650 net/netlink/af_netlink.c:2546
rtnetlink_rcv+0x34/0x40 net/core/rtnetlink.c:6413
netlink_unicast_kernel net/netlink/af_netlink.c:1339 [inline]
netlink_unicast+0xf28/0x1230 net/netlink/af_
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