This is intended for use in KTLS transmit where each TLS record is
described by a single mbuf that is itself queued in the socket buffer.
Using the existing CRYPTO_BUF_MBUF would result in
bus_dmamap_load_crp() walking additional mbufs in the socket buffer
that are not relevant, but generating a S/G list that potentially
exceeds the limit of the tag (while also wasting CPU cycles).
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