pf: Use MTX_DUPOK to initialize hash chain mutexes

pf_udp_mapping_insert() may lock more than one row at a time.

Fixes: cd5ff4e841fb ("pf: use hashalloc(9) for key, id, src-node and udp-endpoint hashes")
Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D56501