The purge thread sets PFI_IFLAG_REFS on the interfaces that states
refer to without the rules lock, under which the other flags are
changed. The updates can interleave, so that a "set skip on" is lost,
or outlives its removal, until the next ruleset load.
Use atomic operations to modify the flags. In the purge thread, only
write if the flag is not already set.
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")