pf: deal with tables gaining or losing counters

When we create a table without counters, add an entry and later
re-define the table to have counters we wound up trying to read
non-existent counters.

We now cope with this by attempting to add them if needed, removing them
when they're no longer needed and not trying to read from counters that
are not present.

MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D34131