pf: Split pf_map_addr()

Split pf_map_addr() into 2 functions:

• pf_map_addr() handles just the address mapping algorithms, it is used in pf_route() and pf_route6() in case of stateless route-to, where source nodes would never be created
• pf_map_addr_sn() handles source nodes and calls pf_map_addr() for address mapping algorightms, it is used everywhere else, like NAT rules, which are always stateful

Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D46776