pf: fix reply-to after rdr and dummynet

If we redirect a packet to localhost and it gets dummynet'd it may be
re-injected later (e.g. when delayed) which means it will be passed
through ip_input() again. ip_input() will then reject the packet because
it's directed to the loopback address, but did not arrive on a loopback
interface.

Fix this by having pf set the rcvif to V_iflo if we redirect to
loopback.

See also: https://redmine.pfsense.org/issues/15363
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