BTRFS decompression benchmark

As a follow-up to [Atemu12's compression benchmark](https://www.reddit.com/r/btrfs/comments/bpphbz/my_benchmarks_of_the_new_zstd_levels_in_51/), here is my measly benchmark of BTRFS decompression speeds.
Same test file, volume in ramfs loop device, compression enforced, caches always dropped, all results best of 10. The ondemand CPU governor was noticeably worse than the performance governor.

Kernel: 5.7.9-1-default (openSUSE Tumbleweed)
Processor: AMD FX-6200
RAM: 4 x 2 GB ECC DDR3-1333

| Setting | Read speed |
|:-|-:|
| btrfs zstd:15 | 974MiB/s |
| btrfs zstd:14 | 985MiB/s |
| btrfs zstd:13 | 1.02GiB/s |
| btrfs zstd:12 | 1.01GiB/s |
| btrfs zstd:11 | 1.02GiB/s |
| btrfs zstd:10 | 1.00GiB/s |
| btrfs zstd:9 | 1.01GiB/s |
| btrfs zstd:8 | 1.01GiB/s |
| btrfs zstd:7 | 1023MiB/s |
| btrfs zstd:6 | 1019MiB/s |
| btrfs zstd:5 | 1008MiB/s |
| btrfs zstd:4 | 1004MiB/s |
| btrfs zstd:3 | 1.02GiB/s |
| btrfs zstd:2 | 1.01GiB/s |
| btrfs zstd:1 | 1.03GiB/s |
| btrfs zlib:3 | 590MiB/s |
| btrfs lzo | 1.31GiB/s |
| btrfs none | 1.49GiB/s |
| xfs | 1.93GiB/s |
| ext4 | 1.93GiB/s |
| ramfs | 3.81GiB/s |

Implementation details:

cpupower frequency-set -g performance
unxz cuda-10.1.168-1-x86_64.pkg.tar.xz
mkdir /mnt/test
mkdir /mnt/ram
mount -t ramfs ramfs /mnt/ram
dd if=/dev/zero of=/mnt/ram/fs.img bs=1M count=6144
losetup /dev/loop0 /mnt/ram/fs.img

mkfs.btrfs -f /dev/loop0
mount -o compress-force=zstd:2 /dev/loop0 /mnt/test
cp cuda-10.1.168-1-x86_64.pkg.tar /mnt/test/
for i in {1..10}; do sync; echo 3 > /proc/sys/vm/drop_caches; nice -n -20 pv -a /mnt/test/cuda-10.1.168-1-x86_64.pkg.tar > /dev/null; done
umount /mnt/test

losetup -d /dev/loop0
umount /mnt/ram

rmdir /mnt/test
rmdir /mnt/ram
rm cuda-10.1.168-1-x86_64.pkg.tar
cpupower frequency-set -g ondemand