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/windows/syscall_windows.go

@@ -10,7 +10,6 @@ import (
 	errorspkg "errors"
 	"fmt"
 	"runtime"
-	"strings"
 	"sync"
 	"syscall"
 	"time"
@@ -87,22 +86,13 @@ func StringToUTF16(s string) []uint16 {
 // s, with a terminating NUL added. If s contains a NUL byte at any
 // location, it returns (nil, syscall.EINVAL).
 func UTF16FromString(s string) ([]uint16, error) {
-	if strings.IndexByte(s, 0) != -1 {
-		return nil, syscall.EINVAL
-	}
-	return utf16.Encode([]rune(s + "\x00")), nil
+	return syscall.UTF16FromString(s)
 }
 
 // UTF16ToString returns the UTF-8 encoding of the UTF-16 sequence s,
 // with a terminating NUL and any bytes after the NUL removed.
 func UTF16ToString(s []uint16) string {
-	for i, v := range s {
-		if v == 0 {
-			s = s[:i]
-			break
-		}
-	}
-	return string(utf16.Decode(s))
+	return syscall.UTF16ToString(s)
 }
 
 // StringToUTF16Ptr is deprecated. Use UTF16PtrFromString instead.