In mb_unmapped_compress() we don't need mbuf structure to keep data,
but we need buffer of MLEN bytes. This isn't just a simplification,
but important fixup, because previous commit shrinked sizeof(struct
mbuf) down below MSIZE, and instantiating an mbuf on stack no longer
provides enough data.

Reviewed by: gallatin
Differential Revision: https://reviews.freebsd.org/D24598