pf: drain Ethernet rules cleanup before starting a new transaction

Inactive Ethernet rules get cleaned by a net_epoch callback. This
callback may still be pending when we try to start a new (pf rules)
transaction, causing it to fail.
This is especially likely to occur in scripted scenarios, such as the
regression tests.

Drain the epoch callbacks before starting a new transaction, ensuring
we've had the opportunity to clean up the inactive rules.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D34846