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/syscall_unix.go

@@ -578,7 +578,7 @@ func Lutimes(path string, tv []Timeval) error {
 	return UtimesNanoAt(AT_FDCWD, path, ts, AT_SYMLINK_NOFOLLOW)
 }
 
-// emptyIovec reports whether there are no bytes in the slice of Iovec.
+// emptyIovecs reports whether there are no bytes in the slice of Iovec.
 func emptyIovecs(iov []Iovec) bool {
 	for i := range iov {
 		if iov[i].Len > 0 {