ipv4: ip_tunnel: Convert ip_tunnel_delete_nets() callers to ->exit_rtnl().

ip_tunnel_delete_nets() iterates the dying netns list and performs the
same operations for each.

Let's export ip_tunnel_destroy() as ip_tunnel_delete_net() and call it
from ->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/20250411205258.63164-7-kuniyu@amazon.com


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