Use the IoctlFileClone and IoctlFileCloneRange ioctl wrappers and the
FileCloneRange type provided by golang.org/x/sys/unix instead of
locally implementing them. This also allows to re-enable the code for
ppc/ppc64/ppc64le again (see commit 758a1a6a37 ("lib/fs: Disable ioctl
on ppc (fixes#6898) (#6901)")) since golang.org/x/sys/unix internally
uses the correct FICLONE and FICLONERANGE values depending on $GOARCH.
