pf: Don't allocate per-table entry counters unless required.

pf by default does not do per-table address accounting unless the
"counters" keyword is specified in the corresponding pf.conf table
definition. Yet, we always allocate 12 per-CPU counters per table. For
large tables this carries a lot of overhead, so only allocate counters
when they will actually be used.

A further enhancement might be to use a dedicated UMA zone to allocate
counter arrays for table entries, since close to half of the structure
size comes from counter pointers. A related issue is the cost of
zeroing counters, since counter_u64_zero() calls smp_rendezvous() on
some architectures.

Reported by: loos, Jim Pingle <jimp@netgate.com>
Reviewed by: kp
MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC (Netgate)
Differential Revision: https://reviews.freebsd.org/D24803