divert: Use CK_SLISTs for the divcb hash table

The hash table is accessed in ip_divert_packet(), and there the accesses
are synchronized only by the net epoch, so plain SLIST is not safe.

Reviewed by: ae
MFC after: 1 week
Sponsored by: OPNsense
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D54011