ip6mr: Lock RCU before ip6mr_get_table() call in ip6mr_new_table()

When IPV6_MROUTE_MULTIPLE_TABLES is enabled, calls to ip6mr_get_table()
must be done under RCU or RTNL lock.

Detected by Lockdep-RCU:

  [   10.247131] WARNING: suspicious RCU usage
  [   10.247133] 6.1.103-49518b10de-nokia_sm_x86 #1 Not tainted
  [   10.247135] -----------------------------
  [   10.247137] /net/ipv6/ip6mr.c:131 RCU-list traversed in non-reader section!!
  [   10.247140]
                 other info that might help us debug this:

  [   10.247142]
                 rcu_scheduler_active = 2, debug_locks = 1
  [   10.247144] 1 lock held by swapper/0/1:
  [   10.247147]  #0: ffffffff82b374d0 (pernet_ops_rwsem){+.+.}-{3:3}, at: register_pernet_subsys+0x15/0x40
  [   10.247164]
                 stack backtrace:
  [   10.247166] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.1.103-49518b10de-nokia_sm_x86 #1
  [   10.247170] Hardware name: Nokia Asil/Default string, BIOS 0ACNA114 07/18/2024
  [   10.247175] Call Trace:
  [   10.247178]  <TASK>
  [   10.247181]  dump_stack_lvl+0xb7/0xe9
  [   10.247189]  lockdep_rcu_suspicious.cold+0x2d/0x64
  [   10.247198]  ip6mr_get_table+0x8a/0x90
  [   10.247203]  ip6mr_net_init+0x7c/0x200
  [   10.247209]  ops_init+0x37/0x1f0
  [   10.247215]  register_pernet_operations+0x129/0x230
  [   10.247221]  ? af_unix_init+0xca/0xca
  [   10.247227]  register_pernet_subsys+0x24/0x40
  [   10.247231]  ip6_mr_init+0x42/0xf2
  [   10.247235]  inet6_init+0x133/0x3b9
  [   10.247238]  do_one_initcall+0x74/0x290
  [   10.247247]  kernel_init_freeable+0x251/0x294
  [   10.247253]  ? rest_init+0x174/0x174
  [   10.247257]  kernel_init+0x16/0x12c
  [   10.247260]  ret_from_fork+0x1f/0x30
  [   10.247271]  </TASK>

Signed-off-by: Stefan Wiehler <[email protected]>
Fixes: d1db275 ("ipv6: ip6mr: support multiple tables")
Signed-off-by: NipaLocal <nipa@local>

net/ipv6/ip6mr.c

@@ -382,7 +382,9 @@ static struct mr_table *ip6mr_new_table(struct net *net, u32 id)
 {
 	struct mr_table *mrt;
 
+	rcu_read_lock();
 	mrt = ip6mr_get_table(net, id);
+	rcu_read_unlock();
 	if (mrt)
 		return mrt;