ipv6: use rcu_dereference_bh() in ipv6_route_seq_next()

This patch replaces rcu_deference() with rcu_dereference_bh() in
ipv6_route_seq_next() to avoid the following warning:

[   19.431685] WARNING: suspicious RCU usage
[   19.433451] 4.14.0-rc3-00914-g66f5d6c #118 Not tainted
[   19.435509] -----------------------------
[   19.437267] net/ipv6/ip6_fib.c:2259 suspicious
rcu_dereference_check() usage!
[   19.440790]
[   19.440790] other info that might help us debug this:
[   19.440790]
[   19.444734]
[   19.444734] rcu_scheduler_active = 2, debug_locks = 1
[   19.447757] 2 locks held by odhcpd/3720:
[   19.449480]  #0:  (&p->lock){+.+.}, at: [<ffffffffb1231f7d>]
seq_read+0x3c/0x333
[   19.452720]  #1:  (rcu_read_lock_bh){....}, at: [<ffffffffb1d2b984>]
ipv6_route_seq_start+0x5/0xfd
[   19.456323]
[   19.456323] stack backtrace:
[   19.458812] CPU: 0 PID: 3720 Comm: odhcpd Not tainted
4.14.0-rc3-00914-g66f5d6c #118
[   19.462042] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS 1.10.2-1 04/01/2014
[   19.465414] Call Trace:
[   19.466788]  dump_stack+0x86/0xc0
[   19.468358]  lockdep_rcu_suspicious+0xea/0xf3
[   19.470183]  ipv6_route_seq_next+0x71/0x164
[   19.471963]  seq_read+0x244/0x333
[   19.473522]  proc_reg_read+0x48/0x67
[   19.475152]  ? proc_reg_write+0x67/0x67
[   19.476862]  __vfs_read+0x26/0x10b
[   19.478463]  ? __might_fault+0x37/0x84
[   19.480148]  vfs_read+0xba/0x146
[   19.481690]  SyS_read+0x51/0x8e
[   19.483197]  do_int80_syscall_32+0x66/0x15a
[   19.484969]  entry_INT80_compat+0x32/0x50
[   19.486707] RIP: 0023:0xf7f0be8e
[   19.488244] RSP: 002b:00000000ffa75d04 EFLAGS: 00000246 ORIG_RAX:
0000000000000003
[   19.491431] RAX: ffffffffffffffda RBX: 0000000000000009 RCX:
0000000008056068
[   19.493886] RDX: 0000000000001000 RSI: 0000000008056008 RDI:
0000000000001000
[   19.496331] RBP: 00000000000001ff R08: 0000000000000000 R09:
0000000000000000
[   19.498768] R10: 0000000000000000 R11: 0000000000000000 R12:
0000000000000000
[   19.501217] R13: 0000000000000000 R14: 0000000000000000 R15:
0000000000000000

Fixes: 66f5d6ce53 ("ipv6: replace rwlock with rcu and spinlock in fib6_table")
Reported-by: Xiaolong Ye <xiaolong.ye@intel.com>
Signed-off-by: Wei Wang <weiwan@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_fib.c

@@ -2262,7 +2262,7 @@ static void *ipv6_route_seq_next(struct seq_file *seq, void *v, loff_t *pos)
 	if (!v)
 		goto iter_table;
 
-	n = rcu_dereference(((struct rt6_info *)v)->dst.rt6_next);
+	n = rcu_dereference_bh(((struct rt6_info *)v)->dst.rt6_next);
 	if (n) {
 		++*pos;
 		return n;