memdesc: Add routines for copying data to/from memory descriptors

These are modeled on the API used for m_copydata/m_copyback and the
crypto buffer APIs. One day it might be nice to reduce the
proliferation of these by adding cursors and using memdesc directly
for crypto request buffers.

Reviewed by: markj
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D40615