pf: Allocate and free pfi_kkif in separate functions

Factor out allocating and freeing pfi_kkif structures. This will be
useful when we change the counters to be counter_u64, so we don't have
to deal with that complexity in the multiple locations where we allocate
pfi_kkif structures.

No functional change.

MFC after: 2 weeks
Sponsored by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D27762