vxlan: mdb: Fix use-after-free in vxlan_mdb_flush()

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

vxlan: mdb: Fix use-after-free in vxlan_mdb_flush()

vxlan_mdb_flush() iterates over the MDB entries using
hlist_for_each_entry_safe(), which only tolerates the removal of the
current entry. Contrary to the comment above the loop, the removal of an
entry can trigger the removal of another entry.

Flushing the remotes of a (*, G) entry also removes the (S, G) entries
that were created for its source list, once they are left without
remotes:

vxlan_mdb_remotes_flush()
-> vxlan_mdb_remote_del()
-> vxlan_mdb_remote_srcs_del()
-> vxlan_mdb_remote_src_del()
-> vxlan_mdb_remote_src_fwd_del()
-> __vxlan_mdb_del()
-> vxlan_mdb_entry_put()

Such an entry can be located after the (*, G) entry in the list, as
vxlan_mdb_entry_get() returns an existing entry without moving it to the
head of the list. This order is obtained by adding the (S, G) entry
before the (*, G) entry, the latter with NLM_F_REPLACE, as the addition
of the source otherwise fails with -EEXIST. The (S, G) entry is then the
entry saved by hlist_for_each_entry_safe() and it is freed while the
(*, G) entry is processed. The next iteration calls hlist_del() on it
again, writing LIST_POISON1 to LIST_POISON2 [1].

Besides device deletion, the flush is also reachable from RTM_DELMDB
with NLM_F_BULK.

Fix by re-reading the next entry after the remotes were flushed. The
current entry cannot be removed by this flush, as source lists can only
be configured on (*, G) entries and the removed entries are (S, G)
entries. It is therefore still linked and its next pointer reflects the
removals.

[1]
BUG: KASAN: wild-memory-access in vxlan_mdb_entry_put.part.0+0x328/0x588
Write of size 8 at addr dead000000000122 by task ip/327

CPU: 3 UID: 1000 PID: 327 Comm: ip Not tainted 7.2.0-rc7 #2 PREEMPT
Call trace:
vxlan_mdb_entry_put.part.0+0x328/0x588
vxlan_mdb_flush+0x1d8/0x25c
vxlan_mdb_fini+0x8c/0x100
vxlan_uninit+0x1c/0x7c
unregister_netdevice_many_notify+0x954/0xd4c
rtnl_dellink+0x210/0x530
rtnetlink_rcv_msg+0x434/0x4d0
netlink_rcv_skb+0xc4/0x204
rtnetlink_rcv+0x18/0x24
netlink_unicast+0x4b8/0x548
netlink_sendmsg+0x29c/0x560
____sys_sendmsg+0x390/0x3ec
___sys_sendmsg+0x114/0x188
__sys_sendmsg+0xf0/0x178
__arm64_sys_sendmsg+0x48/0x60
invoke_syscall.constprop.0+0x58/0x180
el0_svc_common.constprop.0+0x74/0x140
do_el0_svc+0x30/0x40
el0_svc+0x38/0x98
el0t_64_sync_handler+0xa0/0xe4
el0t_64_sync+0x198/0x19c