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/release/Dockerfile

@@ -28,7 +28,6 @@ RUN	apt-get update && \
 	apt-get install -y dpkg-dev git make pkg-config
 ARG TARGETPLATFORM
 RUN xx-apt-get install -y libseccomp-dev btrfs-progs gcc
-RUN if grep -qE 'UBUNTU_CODENAME=(focal|jammy)' /etc/os-release; then xx-apt-get install -y libbtrfs-dev; fi
 ENV PATH=/usr/local/go/bin:$PATH
 ENV GOPATH=/go
 ENV CGO_ENABLED=1