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

sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

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

memdesc: Retire MEMDESC_BIO.

Instead, change memdesc_bio to examine the bio and return a memdesc of
a more generic type describing the data buffer.

Reviewed by: imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D41029

memdesc: Add a MEMDESC_VMPAGES descriptor type.

This memory descriptor is backed by an array of VM pages. This type
requires adding a new field to 'struct memdesc' to hold the offset
within the first page. For LP64 systems, this new field is added in
an existing padding hole so does not increase the size. For ILP32
systems, this grows 'struct memdesc' from 12 to 16 bytes.

Reviewed by: imp, markj
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D41028

memdesc: Replace md_opaque with a union of type-specific fields.

Reviewed by: imp, markj
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D41027

memdesc: Retire MEMDESC_CCB.

Instead, change memdesc_ccb to examine the CCB and return a memdesc of
a more generic type describing the data buffer.

Reviewed by: imp, markj
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D40880

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

sys/sys: further adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 2-Clause license, however the tool I
was using misidentified many licenses so this was mostly a manual - error
prone - task.

The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.

Reform the busdma API so that new types may be added without modifying
every architecture's busdma_machdep.c. It is done by unifying the
bus_dmamap_load_buffer() routines so that they may be called from MI
code. The MD busdma is then given a chance to do any final processing
in the complete() callback.

The cam changes unify the bus_dmamap_load* handling in cam drivers.

The arm and mips implementations are updated to track virtual
addresses for sync(). Previously this was done in a type specific
way. Now it is done in a generic way by recording the list of
virtuals in the map.

Submitted by: jeff (sponsored by EMC/Isilon)
Reviewed by: kan (previous version), scottl,
mjacob (isp(4), no objections for target mode changes)
Discussed with: ian (arm changes)
Tested by: marius (sparc64), mips (jmallet), isci(4) on x86 (jharris),
amd64 (Fabian Keil <freebsd-listen@fabiankeil.de>)