ipv6: Convert tunnel devices' ->exit_batch_rtnl() to ->exit_rtnl().

ipv6: Convert tunnel devices' ->exit_batch_rtnl() to ->exit_rtnl().

The following functions iterates the dying netns list and performs
the same operations for each netns.

  * ip6gre_exit_batch_rtnl()
  * ip6_tnl_exit_batch_rtnl()
  * vti6_exit_batch_rtnl()
  * sit_exit_batch_rtnl()

Let's use ->exit_rtnl().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/20250411205258.63164-8-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>