CVE-2026-98881: net: bridge: remove stale rcu_barrier() in br_multicast_dev_del()

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

net: bridge: remove stale rcubarrier() in brmulticastdevdel()

This rcubarrier() came from a time callrcu() calls were used in net/bridge/brmulticast.c.

Now kfreercu() is there, we can remove this problematic rcubarrier() which causes extreme RTNL pressure in many syzbot reports.

INFO: task syz-executor:77945 is blocked on a mutex likely owned by task kworker/u1024:5:36537. task:kworker/u1024:5 state:D stack:24616 pid:36537 tgid:36537 ppid:2 taskflags:0x4208060 flags:0x00080000 lastsleep:612797637337 Workqueue: netns cleanupnet Call Trace: <TASK> [<ffffffff81914eaa>] contextswitch+0xf2a/0x1730 kernel/sched/core.c:6483 [<ffffffff81201143>] schedule+0x1133/0x43a0 kernel/sched/core.c:8411 [<ffffffff8120446b>] scheduleloop kernel/sched/core.c:8514 [inline] [<ffffffff8120446b>] schedule+0xab/0x260 kernel/sched/core.c:8529 [<ffffffff8121a093>] scheduletimeout+0xc3/0x2b0 kernel/time/sleeptimeout.c:75 [<ffffffff81205347>] dowaitforcommon kernel/sched/completion.c:100 [inline] [<ffffffff81205347>] waitforcommon kernel/sched/completion.c:121 [inline] [<ffffffff81205347>] waitforcommon kernel/sched/completion.c:132 [inline] [<ffffffff81205347>] waitforcompletion+0x2c7/0x5d0 kernel/sched/completion.c:153 [<ffffffff81b8f27f>] rcubarrier+0x49f/0x620 kernel/rcu/tree.c:3888 [<ffffffff860091b3>] brmulticastdevdel+0x303/0x350 net/bridge/brmulticast.c:4459 [<ffffffff85fb5dbc>] brdevuninit+0x1c/0x40 net/bridge/brdevice.c:157 [<ffffffff8568058c>] unregisternetdevicemanynotify+0x1c1c/0x2300 net/core/dev.c:12599 [<ffffffff8562be43>] opsexitrtnllist net/core/netnamespace.c:187 [inline] [<ffffffff8562be43>] opsundolist+0x3d3/0x940 net/core/netnamespace.c:248