crypto: Add a new type of crypto buffer for a single mbuf.

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).

Reviewed by: markj
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D30136