- Use the carp_sx to serialize not only CARP ioctls, but also carp_attach()
and carp_detach().
- Use cif_mtx to lock only access to those the linked list.
- These locking changes allow us to do some memory allocations with M_WAITOK
and also properly call callout_drain() in carp_destroy().
- In carp_attach() assert that ifaddr isn't attached. We always come here
with a pristine address from in[6]_control().
Reviewed by: oleg
Sponsored by: Nginx, Inc.
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