Use the CopyFileRange and Lsetxattr from golang.org/x/sys/unix instead
of their counterparts from github.com/containerd/continuity/sysx. These
are 1:1 replacements (except for the fd parameter types in
CopyFileRange).
This will eventually allow to remove these functions there as well.
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>