Backport btrfs enospc rework (from kernel 4.8)

There are two btrfs related patches available for kernel 4.8 which improve latency and solve problems with low free disk space:

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=ba929b6646c5b87c7bb15cd8d3e51617725c983b

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=d58b0d980f0b1c27204da0e05537b606da45a07f

Seen at (german source): http://www.heise.de/ct/artikel/Die-Neuerungen-von-Linux-4-8-3283402.html

From my side, I encountered problems with full disks in btrfs multiple times. I reported some of them in the past.

Plus do I have another system with 2x3TB of disk space (WD HDDs), while btrfs sits on top of a dm-raid and LUKS crypto. I experience very slow disk I/O on that system while disk usage gets fuller and fuller (combined with lots of subvolumes and layered devices, so that could be another reason).

That said, I guess that, according to the article at heise and commit messages, the two patches have a tremendous impact on performance. Plus, should the patches fix some of the problems that happen together with full disks ("ENOSPC" issues).

Now that the patches make it into the upstream kernel, I think that the developers have had some reason to do so. For the moment, I can't test if it actually improves something for me.

Would still be great to have these patches backported to
- ubuntu 16.04 Kernel 4.4
- ubuntu 14.04 LTS Kernel 4.4

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: linux-image-4.4.0-36-generic 4.4.0-36.55
ProcVersionSignature: Ubuntu 4.4.0-36.55-generic 4.4.16
Uname: Linux 4.4.0-36-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: thomas 4788 F.... pulseaudio
CurrentDesktop: GNOME-Flashback:Unity
Date: Sun Sep 11 07:39:13 2016
HibernationDevice: RESUME=UUID=28eded29-f9e6-44b5-9e54-c4dd60c8e3cd
InstallationDate: Installed on 2014-11-29 (652 days ago)
InstallationMedia: Ubuntu 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.2)
MachineType: Dell Inc. Latitude E5510
ProcFB: 0 inteldrmfb
ProcKernelCmdLine: BOOT_IMAGE=/@/boot/vmlinuz-4.4.0-36-generic root=UUID=6408c2d9-1b60-43d7-9a7f-2dceeb40de28 ro rootflags=subvol=@ quiet splash vt.handoff=7
RelatedPackageVersions:
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 linux-backports-modules-4.4.0-36-generic N/A
 linux-firmware 1.157.3
SourcePackage: linux
UpgradeStatus: Upgraded to xenial on 2016-06-15 (87 days ago)
dmi.bios.date: 12/06/2013
dmi.bios.vendor: Dell Inc.
dmi.bios.version: A16
dmi.board.name: 023HKR
dmi.board.vendor: Dell Inc.
dmi.board.version: A00
dmi.chassis.type: 9
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvrA16:bd12/06/2013:svnDellInc.:pnLatitudeE5510:pvr0001:rvnDellInc.:rn023HKR:rvrA00:cvnDellInc.:ct9:cvr:
dmi.product.name: Latitude E5510
dmi.product.version: 0001
dmi.sys.vendor: Dell Inc.