memdesc: Helper function to construct mbuf chain backed by memdesc buffer

memdesc_alloc_ext_mbufs constructs a chain of external (M_EXT or
M_EXTPG) mbufs backed by a data buffer described by a memory
descriptor.

Since memory descriptors are not an actual buffer just a description
of a buffer, the caller is required to supply a couple of helper
routines to manage allocation of the raw mbufs and associating them
with a reference to the underlying buffer.

Reviewed by: markj
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D42933

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