LinuxKPI: add addrconf_addr_solict_mult()

Introduce net/addrconf.h with an implementation to
addrconf_addr_solict_mult() used by WiFi drivers.

Sponsored by: The FreeBSD Foundation
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D30416

(cherry picked from commit 095f018e4915bf63447ecd6be3beae0dd1a5ff52)