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