pf: fix dummynet + ipdivert use case

[igoro]

Summary

Dummynet re-injects an mbuf with MTAG_IPFW_RULE added, and the same mtag
is used by divert(4) as parameters for packet diversion.

If according to pf rule set a packet should go through dummynet first
and through ipdivert after then mentioned mtag must be removed after
dummynet not to make ipdivert think that this is its input parameters.

At the very beginning ipfw consumes this mtag what means the same
behavior with tag clearing after dummynet.

And after fabf705f4b5a pf passes parameters to ipdivert using its
personal MTAG_PF_DIVERT mtag.
    
PR:             274850

[kp]

This seems sane to me (and the existing tests pass, which is a nice thing to have).

Two minor remarks, more about taste than actual issues.

sys/netpfil/pf/pf.c

306

I'd call that pf_dummynet_flag_remove() or something

tests/sys/netpfil/pf/Makefile

11

I'd put this in the existing divert-to tests file.

[igoro]

Sure, it makes sense. Please, consider the second version of the patch.