ipmr: Move unregister_netdevice_many() out of mroute_clean_tables().

ipmr: Move unregister_netdevice_many() out of mroute_clean_tables().

This is a prep commit to convert ipmr_net_exit_batch() to
->exit_rtnl().

Let's move unregister_netdevice_many() in mroute_clean_tables()
to its callers.

As a bonus, mrtsock_destruct() can do batching for all tables.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260228221800.1082070-8-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>