dummynet: fix codel

Serialize rcvif when enqueing packets for codel. We already tried to
restore the serialized rcvif in fq_codel_extract_head(), but that
doesn't work when we fail to serialize it first, so we ended up dropping
all packets passed through codel.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D37318