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>

vendor/golang.org/x/sys/cpu/cpu_linux.go

@@ -0,0 +1,16 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !386 && !amd64 && !amd64p32 && !arm64
+// +build !386,!amd64,!amd64p32,!arm64
+
+package cpu
+
+func archInit() {
+	if err := readHWCAP(); err != nil {
+		return
+	}
+	doinit()
+	Initialized = true
+}