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/zsyscall_linux.go

@@ -537,6 +537,17 @@ func Chroot(path string) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func ClockAdjtime(clockid int32, buf *Timex) (state int, err error) {
+	r0, _, e1 := Syscall(SYS_CLOCK_ADJTIME, uintptr(clockid), uintptr(unsafe.Pointer(buf)), 0)
+	state = int(r0)
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func ClockGetres(clockid int32, res *Timespec) (err error) {
 	_, _, e1 := Syscall(SYS_CLOCK_GETRES, uintptr(clockid), uintptr(unsafe.Pointer(res)), 0)
 	if e1 != 0 {