pf: fix packet-to-big for route-to as well

When we handle a packet via route-to (i.e. pf_route6()) we still need to
verify the MTU. However, we only run that check in the forwarding case.

Set the PFIL_FWD tag when running the pf_test6(PF_OUT) check from
pf_route6(). We are in fact forwarding, so should call the test function
as such. This will cause us to run the MTU check, and generate an ICMP6
packet-too-big error when required.

See also: 54c62e3e5d8cd90c5571a1d4c8c5f062d580480e
See also: f1c0030bb05cfa01bdd500e50befbb425fecc4c4
See also: https://redmine.pfsense.org/issues/14290
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