ppp: Split ppp_exit_net() to ->exit_rtnl().

ppp: Split ppp_exit_net() to ->exit_rtnl().

ppp_exit_net() unregisters devices related to the netns under
RTNL and destroys lists and IDR.

Let's use ->exit_rtnl() for the device unregistration part to
save RTNL dances for each netns.

Note that we delegate the for_each_netdev_safe() part to
default_device_exit_batch() and replace unregister_netdevice_queue()
with ppp_nl_dellink() to align with bond, geneve, gtp, and pfcp.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250418003259.48017-4-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>