ipmr: use READ_ONCE() to read net->ipv[46].ipmr_seq

ipmr: use READ_ONCE() to read net->ipv[46].ipmr_seq

mr_call_vif_notifiers() and mr_call_mfc_notifiers() already
uses WRITE_ONCE() on the write side.

Using RTNL to protect the reads seems a big hammer.

Constify 'struct net' argument of ip6mr_rules_seq_read()
and ipmr_rules_seq_read().

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20241009184405.3752829-5-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>