bridge: fix gc_timer mod/del race condition

commit c62987bb ("bridge: push bridge setting ageing_time down to
switchdev") introduced a timer race condition because the gc_timer can
get rearmed after it's supposedly stopped and flushed in br_dev_delete()
leading to a use of freed memory. So take rtnl to sync with bridge
destruction when setting ageing_timer.
Here's the trace reproduced with these two commands running in parallel:
while :; do echo 10000 > /sys/class/net/br0/bridge/ageing_timer; done;
while :; do brctl addbr br0; ip l set br0 up; ip l set br0 down;
brctl delbr br0; done;

[  300.000029] BUG: unable to handle kernel paging request at
ffffffff811c59d3
[  300.000263] IP: [<ffffffff810f168e>] __internal_add_timer+0x2e/0xd0
[  300.000422] PGD 1a0f067 PUD 1a10063 PMD 10001e1
[  300.000639] Oops: 0003 [#1] SMP
[  300.000793] Modules linked in: bridge stp llc nfsd auth_rpcgss
oid_registry nfs_acl nfs lockd grace fscache sunrpc crct10dif_pclmul
crc32_pclmul crc32c_intel ghash_clmulni_intel ppdev aesni_intel
aes_x86_64 glue_helper lrw gf128mul ablk_helper cryptd
snd_hda_codec_generic qxl drm_kms_helper psmouse pcspkr ttm
snd_hda_intel 9pnet_virtio evdev serio_raw joydev snd_hda_codec 9pnet
virtio_balloon drm snd_hwdep virtio_console snd_hda_core pvpanic snd_pcm
i2c_piix4 snd_timer acpi_cpufreq parport_pc snd parport soundcore button
processor i2c_core ipv6 autofs4 hid_generic usbhid hid ext4 crc16
mbcache jbd2 sg sr_mod cdrom ata_generic virtio_blk virtio_net e1000
ehci_pci uhci_hcd ehci_hcd usbcore usb_common floppy ata_piix libata
virtio_pci virtio_ring virtio scsi_mod
[  300.004008] CPU: 1 PID: 1169 Comm: bash Not tainted 4.3.0-rc3+ #46
[  300.004008] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[  300.004008] task: ffff880035be2200 ti: ffff88003795c000 task.ti:
ffff88003795c000
[  300.004008] RIP: 0010:[<ffffffff810f168e>]  [<ffffffff810f168e>]
__internal_add_timer+0x2e/0xd0
[  300.004008] RSP: 0018:ffff88003fd03e78  EFLAGS: 00010046
[  300.004008] RAX: ffff88003fd0ef60 RBX: 840fc78949c08548 RCX:
00000001ffffffff
[  300.004008] RDX: 0000000000000000 RSI: ffffffff811c59d3 RDI:
ffff88003fd0df00
[  300.004008] RBP: ffff88003fd03e78 R08: 00000000ffffffff R09:
0000000000000000
[  300.004008] R10: 0000000000000000 R11: 0000000000000000 R12:
ffff88003fd0df00
[  300.004008] R13: 0000000000000000 R14: 0000000000000001 R15:
ffffffff816032e0
[  300.004008] FS:  00007fcbdd609700(0000) GS:ffff88003fd00000(0000)
knlGS:0000000000000000
[  300.004008] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  300.004008] CR2: ffffffff811c59d3 CR3: 0000000037879000 CR4:
00000000000406e0
[  300.004008] Stack:
[  300.004008]  ffff88003fd03ea8 ffffffff810f1775 ffff88003c8cb958
ffff88003fd0df00
[  300.004008]  0000000000000000 0000000000000001 ffff88003fd03f18
ffffffff810f28c4
[  300.004008]  ffff88003fd0eb68 ffff88003fd0e968 ffff88003fd0e768
ffff88003fd0df68
[  300.004008] Call Trace:
[  300.004008]  <IRQ>
[  300.004008]  [<ffffffff810f1775>] cascade+0x45/0x70
[  300.004008]  [<ffffffff810f28c4>] run_timer_softirq+0x2f4/0x340
[  300.004008]  [<ffffffff8107e380>] __do_softirq+0xd0/0x440
[  300.004008]  [<ffffffff8107e8a3>] irq_exit+0xb3/0xc0
[  300.004008]  [<ffffffff815c2032>] smp_apic_timer_interrupt+0x42/0x50
[  300.004008]  [<ffffffff815bfe37>] apic_timer_interrupt+0x87/0x90
[  300.004008]  <EOI>
[  300.004008]  [<ffffffff811fb80c>] ? create_object+0x13c/0x2e0
[  300.004008]  [<ffffffff8109b23e>] ? __kernel_text_address+0x4e/0x70
[  300.004008]  [<ffffffff8109b23e>] ? __kernel_text_address+0x4e/0x70
[  300.004008]  [<ffffffff8101e17f>] print_context_stack+0x7f/0xf0
[  300.004008]  [<ffffffff8101d55b>] dump_trace+0x11b/0x300
[  300.004008]  [<ffffffff8102970b>] save_stack_trace+0x2b/0x50
[  300.004008]  [<ffffffff811fb80c>] create_object+0x13c/0x2e0
[  300.004008]  [<ffffffff815b2e8e>] kmemleak_alloc+0x4e/0xb0
[  300.004008]  [<ffffffff811e475d>] kmem_cache_alloc_trace+0x18d/0x2f0
[  300.004008]  [<ffffffff8128b139>] kernfs_fop_open+0xc9/0x380
[  300.004008]  [<ffffffff8120214f>] do_dentry_open+0x1ff/0x2f0
[  300.004008]  [<ffffffff8128b070>] ? kernfs_fop_release+0x70/0x70
[  300.004008]  [<ffffffff812034f9>] vfs_open+0x59/0x60
[  300.004008]  [<ffffffff812130de>] path_openat+0x1ce/0x1260
[  300.004008]  [<ffffffff812154ae>] do_filp_open+0x7e/0xe0
[  300.004008]  [<ffffffff812251ff>] ? __alloc_fd+0xaf/0x180
[  300.004008]  [<ffffffff8120387b>] do_sys_open+0x12b/0x210
[  300.004008]  [<ffffffff8120397e>] SyS_open+0x1e/0x20
[  300.004008]  [<ffffffff815bf0b6>] entry_SYSCALL_64_fastpath+0x16/0x7a
[  300.004008] Code: 66 90 48 8b 46 10 48 8b 4f 40 55 48 89 c2 48 89 e5
48 29 ca 48 81 fa ff 00 00 00 77 20 0f b6 c0 48 8d 44 c7 68 48 8b 10 48
85 d2 <48> 89 16 74 04 48 89 72 08 48 89 30 48 89 46 08 5d c3 48 81 fa
[  300.004008] RIP  [<ffffffff810f168e>] __internal_add_timer+0x2e/0xd0
[  300.004008]  RSP <ffff88003fd03e78>
[  300.004008] CR2: ffffffff811c59d3

Fixes: c62987bb ("bridge: push bridge setting ageing_time down to switchdev")
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: Scott Feldman <sfeldma@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/bridge/br_sysfs_br.c

@@ -102,7 +102,15 @@ static ssize_t ageing_time_show(struct device *d,
 
 static int set_ageing_time(struct net_bridge *br, unsigned long val)
 {
-	return br_set_ageing_time(br, val);
+	int ret;
+
+	if (!rtnl_trylock())
+		return restart_syscall();
+
+	ret = br_set_ageing_time(br, val);
+	rtnl_unlock();
+
+	return ret;
 }
 
 static ssize_t ageing_time_store(struct device *d,