sys: Remove $FreeBSD$: one-line .c comment pattern

Remove /^/[*/]\s*\$FreeBSD\$.*\n/

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

Don't import 0 into vmem quantum caches.

vmem uses UMA cache zones to implement the quantum cache. Since
uma_zalloc() returns 0 (NULL) to signal an allocation failure, UMA
should not be used to cache resource 0. Fix this by ensuring that 0 is
never cached in UMA in the first place, and by modifying vmem_alloc()
to fall back to a search of the free lists if the cache is depleted,
rather than blocking in qc_import().

Reported by and discussed with: Brett Gutstein <bgutstein@rice.edu>
Reviewed by: alc
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D17483

Eliminate kmem_arena and kmem_object in preparation for further NUMA commits.

The arena argument to kmem_*() is now only used in an assert. A follow-up
commit will remove the argument altogether before we freeze the API for the
next release.

This replaces the hard limit on kmem size with a soft limit imposed by UMA. When
the soft limit is exceeded we periodically wakeup the UMA reclaim thread to
attempt to shrink KVA. On 32bit architectures this should behave much more
gracefully as we exhaust KVA. On 64bit the limits are likely never hit.

Reviewed by: markj, kib (some objections)
Discussed with: alc
Tested by: pho
Sponsored by: Netflix / Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D13187

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.

Make ZFS ARC track both KVA usage and fragmentation.

Even on Illumos, with its much larger KVA, ZFS ARC steps back if KVA usage
reaches certain threshold (3/4 on i386 or 16/17 otherwise). FreeBSD has
even less KVA, but had no such limit on archs with direct map as amd64.
As result, on machines with a lot of RAM, during load with very small user-
space memory pressure, such as `zfs send`, it was possible to reach state,
when there is enough both physical RAM and KVA (I've seen up to 25-30%),
but no continuous KVA range to allocate even single 128KB I/O request.

Address this situation from two sides:
- restore KVA usage limitations in a way the most close to Illumos;
- introduce new requirement for KVA fragmentation, specifying that we
should have at least one sequential KVA range of zfs_max_recordsize bytes.

Experiments show that first limitation done alone is not sufficient. On
machine with 64GB of RAM it is sometimes needed to drop up to half of ARC
size to get at leats one 1MB KVA chunk. Statically limiting ARC to half
of KVA/RAM is too strict, so second limitation makes it to work in cycles:
accumulate trash up to certain critical mass, do massive spring-cleaning,
and then start littering again. :)

MFC after: 1 month

- Add a general purpose resource allocator, vmem, from NetBSD. It was
originally inspired by the Solaris vmem detailed in the proceedings
of usenix 2001. The NetBSD version was heavily refactored for bugs
and simplicity.
- Use this resource allocator to allocate the buffer and transient maps.
Buffer cache defrags are reduced by 25% when used by filesystems with
mixed block sizes. Ultimately this may permit dynamic buffer cache
sizing on low KVA machines.

Discussed with: alc, kib, attilio
Tested by: pho
Sponsored by: EMC / Isilon Storage Division