pfctl: Call ifa_load() before ifa_grouplookup()

ifa_grouplookup() uses the data loaded in ifa_load() (through is_a_group()), so
we must call ifa_load() before we can rely on any of the data it populates.

Submitted by: Nick Rogers
MFC after: 1 week
Sponsored by: RG Nets

(cherry picked from commit 8fd675d8a0538544a85483f7a9bed2bd48382926)