ip6mr: Move unregister_netdevice_many() out of ip6mr_free_table().

ip6mr: Move unregister_netdevice_many() out of ip6mr_free_table().

This is a prep commit to convert ip6mr_net_exit_batch() to
->exit_rtnl().

Let's move unregister_netdevice_many() in ip6mr_free_table()
to its callers.

Now ip6mr_rules_exit() can do batching all tables per netns.

Note that later we will remove RTNL and unregister_netdevice_many()
in ip6mr_rules_init().

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260604224712.3209821-12-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>