pf: Support killing floating states by interface

Floating states get assigned to interface 'all' (V_pfi_all), so when we
try to flush all states for an interface states originally created
through this interface are not flushed. Only if-bound states can be
flushed in this way.

Given that we track the original interface we can check if the state's
interface is 'all', and if so compare to the orig_if instead.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D30246