btrfs: depend on kernel UAPI instead of libbtrfs

See containerd/btrfs PR 40 and moby/moby PR 44761. (Thanks to [@]neersighted.)

The containerd/btrfs library now requires headers from kernel 4.12 or newer:
- https://github.com/torvalds/linux/blob/master/include/uapi/linux/btrfs.h
- https://github.com/torvalds/linux/blob/master/include/uapi/linux/btrfs_tree.h

These files are licensed under the GPL-2.0 WITH Linux-syscall-note, so it should be compatible with the Apache License 2.0.
https://spdx.org/licenses/Linux-syscall-note.html

The dependency on the kernel headers only affects users building from source.
Users on older kernels may opt to not compile this library (`BUILDTAGS=no_btfs`),
or to provide headers from a newer kernel.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

.github/workflows/ci.yml

@@ -34,12 +34,6 @@ jobs:
         os: [ubuntu-20.04, macos-12, windows-2019]
 
     steps:
-      - name: Install dependencies
-        if: matrix.os == 'ubuntu-20.04'
-        run: |
-          sudo apt-get update
-          sudo apt-get install -y libbtrfs-dev
-
       - uses: actions/setup-go@v3
         with:
           go-version: ${{ env.GO_VERSION }}
@@ -165,7 +159,7 @@ jobs:
       - run: |
           set -e -x
 
-          packages="libbtrfs-dev"
+          packages=""
           platform="${{matrix.goos}}/${{matrix.goarch}}"
           if [ -n "${{matrix.goarm}}" ]; then
             platform+="/v${{matrix.goarm}}"
@@ -214,12 +208,6 @@ jobs:
         os: [ubuntu-20.04, macos-12, windows-2019, windows-2022]
         go-version: ["1.19.5", "1.18.10"]
     steps:
-      - name: Install dependencies
-        if: matrix.os == 'ubuntu-20.04'
-        run: |
-          sudo apt-get update
-          sudo apt-get install -y libbtrfs-dev
-
       - uses: actions/setup-go@v3
         with:
           go-version: ${{ matrix.go-version }}
@@ -437,7 +425,7 @@ jobs:
           RUNC_FLAVOR: ${{ matrix.runc }}
         run: |
           sudo apt-get update
-          sudo apt-get install -y gperf libbtrfs-dev
+          sudo apt-get install -y gperf
           script/setup/install-seccomp
           script/setup/install-runc
           script/setup/install-cni $(grep containernetworking/plugins go.mod | awk '{print $2}')