pfcp: Convert pfcp_net_exit() to ->exit_rtnl().

pfcp: Convert pfcp_net_exit() to ->exit_rtnl().

pfcp_net_exit() holds RTNL and cleans up all devices in the netns
and other devices tied to sockets in the netns.

We can use ->exit_rtnl() to save RTNL dance for all dying netns.

Note that we delegate the for_each_netdev() part to
default_device_exit_batch() to avoid a list corruption splat
like the one reported in commit 4ccacf86491d ("gtp: Suppress
list corruption splat in gtp_net_exit_batch_rtnl().")

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250418003259.48017-3-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>