netns list (just list) is under RTNL. But helper and proto unregistration
happen during rmmod when RTNL is not held, and that's how it was tested:
modprobe/rmmod vs clone(CLONE_NEWNET)/exit.

BUG: unable to handle kernel paging request at 0000000000100100	<===
IP: [<ffffffffa009890f>] nf_conntrack_l4proto_unregister+0x96/0xae [nf_conntrack]
PGD 15e300067 PUD 15e1d8067 PMD 0
Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
last sysfs file: /sys/kernel/uevent_seqnum
CPU 0
Modules linked in: nf_conntrack_proto_sctp(-) nf_conntrack_proto_dccp(-) af_packet iptable_nat nf_nat nf_conntrack_ipv4 nf_conntrack nf_defrag_ipv4 iptable_filter ip_tables xt_tcpudp ip6table_filter ip6_tables x_tables ipv6 sr_mod cdrom [last unloaded: nf_conntrack_proto_sctp]
Pid: 16758, comm: rmmod Not tainted 2.6.28-rc2-netns-xfrm #3
RIP: 0010:[<ffffffffa009890f>]  [<ffffffffa009890f>] nf_conntrack_l4proto_unregister+0x96/0xae [nf_conntrack]
RSP: 0018:ffff88015dc1fec8  EFLAGS: 00010212
RAX: 0000000000000000 RBX: 00000000001000f8 RCX: 0000000000000000
RDX: ffffffffa009575c RSI: 0000000000000003 RDI: ffffffffa00956b5
RBP: ffff88015dc1fed8 R08: 0000000000000002 R09: 0000000000000000
R10: 0000000000000000 R11: ffff88015dc1fe48 R12: ffffffffa0458f60
R13: 0000000000000880 R14: 00007fff4c361d30 R15: 0000000000000880
FS:  00007f624435a6f0(0000) GS:ffffffff80521580(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 0000000000100100 CR3: 0000000168969000 CR4: 00000000000006e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Process rmmod (pid: 16758, threadinfo ffff88015dc1e000, task ffff880179864218)
Stack:
 ffffffffa0459100 0000000000000000 ffff88015dc1fee8 ffffffffa0457934
 ffff88015dc1ff78 ffffffff80253fef 746e6e6f635f666e 6f72705f6b636172
 00707463735f6f74 ffffffff8024cb30 00000000023b8010 0000000000000000
Call Trace:
 [<ffffffffa0457934>] nf_conntrack_proto_sctp_fini+0x10/0x1e [nf_conntrack_proto_sctp]
 [<ffffffff80253fef>] sys_delete_module+0x19f/0x1fe
 [<ffffffff8024cb30>] ? trace_hardirqs_on_caller+0xf0/0x114
 [<ffffffff803ea9b2>] ? trace_hardirqs_on_thunk+0x3a/0x3f
 [<ffffffff8020b52b>] system_call_fastpath+0x16/0x1b
Code: 13 35 e0 e8 c4 6c 1a e0 48 8b 1d 6d c6 46 e0 eb 16 48 89 df 4c 89 e2 48 c7 c6 fc 85 09 a0 e8 61 cd ff ff 48 8b 5b 08 48 83 eb 08 <48> 8b 43 08 0f 18 08 48 8d 43 08 48 3d 60 4f 50 80 75 d3 5b 41
RIP  [<ffffffffa009890f>] nf_conntrack_l4proto_unregister+0x96/0xae [nf_conntrack]
 RSP <ffff88015dc1fec8>
CR2: 0000000000100100
---[ end trace bde8ac82debf7192 ]---

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>