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/unix/xattr_bsd.go

@@ -36,9 +36,14 @@ func xattrnamespace(fullattr string) (ns int, attr string, err error) {
 func initxattrdest(dest []byte, idx int) (d unsafe.Pointer) {
 	if len(dest) > idx {
 		return unsafe.Pointer(&dest[idx])
-	} else {
-		return unsafe.Pointer(_zero)
 	}
+	if dest != nil {
+		// extattr_get_file and extattr_list_file treat NULL differently from
+		// a non-NULL pointer of length zero. Preserve the property of nilness,
+		// even if we can't use dest directly.
+		return unsafe.Pointer(&_zero)
+	}
+	return nil
 }
 
 // FreeBSD and NetBSD implement their own syscalls to handle extended attributes