net: Add ops_undo_single for module load/unload.

net: Add ops_undo_single for module load/unload.

If ops_init() fails while loading a module or we unload the
module, free_exit_list() rolls back the changes.

The rollback sequence is the same as ops_undo_list().

The ops is already removed from pernet_list before calling
free_exit_list().  If we link the ops to a temporary list,
we can reuse ops_undo_list().

Let's add a wrapper of ops_undo_list() and use it instead
of free_exit_list().

Now, we have the central place to roll back ops_init().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/20250411205258.63164-3-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>