bridge: Convert br_net_exit_batch_rtnl() to ->exit_rtnl().

br_net_exit_batch_rtnl() iterates the dying netns list and
performs the same operation for each.

Let's use ->exit_rtnl().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/20250411205258.63164-10-kuniyu@amazon.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>