Use the common codepath to handle SIOCGIFADDR.

Before this change, the current code handles SIOCGIFADDR the same
way with SIOCSIFADDR, which involves full arp_ifinit, et al. They
should be unnecessary for SIOCGIFADDR case.

Differential Revision: https://reviews.freebsd.org/D1508
Reviewed by: glebius
MFC after: 2 weeks