pf: Add static DTrace probe points

These two have proven to be useful during debugging. We may as well keep
them permanently.
Others will be added as their utility becomes clear.

Reviewed by: gnn
MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29555

(cherry picked from commit f4c02909167b6f791df470afddfe31cabf009c4d)