ipmr: Call fib_rules_unregister() without RTNL.

ipmr: Call fib_rules_unregister() without RTNL.

fib_rules_unregister() removes ops from net->rules_ops under
spinlock, calls ops->delete() for each rule, and frees the ops.

ipmr_rules_ops_template does not have ->delete(), and any
operation does not require RTNL there.

Let's move fib_rules_unregister() from ipmr_rules_exit_rtnl()
to ipmr_net_exit().

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260228221800.1082070-12-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>