Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC: r258137
Introduce allocation cache to store LZ4 compression contexts without kicking
VM subsystem twice for every written record.

Tests on 24-core system show double reduction of CPU time spent on copying
single large well-compressed file.

This patch is not really needed on illumos (while not harm either) since
their memory allocator by default uses caching for all requests up to 128K.

MFC r246586,246587,246619,246624,246768,246808:

LZ4 compression support in ZFS.

(Note: sys/conf/files change omitted from this changeset).

MFV r245512:

* Illumos zfs issue #3035 [1] LZ4 compression support in ZFS.

LZ4 is a new high-speed BSD-licensed compression algorithm created
by Yann Collet that delivers very high compression and decompression
performance compared to lzjb (>50% faster on compression, >80% faster
on decompression and around 3x faster on compression of incompressible
data), while giving better compression ratio [1].

This version of LZ4 corresponds to upstream's [2] revision 85.

Please note that for obvious reasons this is not backward read
compatible. This means once a pool have LZ4 compressed data, these
data can no longer be read by older ZFS implementations.

Local changes:

- On-stack hash table disabled and using kernel slab allocator
instead, at this time. This requires larger kernel thread stack
for zio workers. This may change in the future should we adjusted
the zio workers' thread stack size.
- likely and unlikely will be undefined if they are already defined,
this is required for i386 XEN build.
- Removed De Bruijn sequence based __builtin_ctz family of builtins
in favor of the latter. Both GCC and clang supports these builtins.
- Changed the way the LZ4 code detects endianness.
- Manual pages modifications to mention the feature based on Illumos
counterpart.
- Boot loader changes to make it support LZ4 decompression.

[1] https://www.illumos.org/issues/3035
[2] http://code.google.com/p/lz4/source/list

Obtained from: Illumos (13921:9d721847e469)
Tested on: FreeBSD/amd64
MFC after: 1 month

Update vendor/illumos/dist and vendor-sys/illumos/dist
to illumos-gate 13921:9d721847e469
(illumos zfs issue #3035 LZ4 compression support in ZFS and GRUB)