./pkg/archive: clean up Stat_t assertion

inspired by @tonistiigi comment
(https://github.com/docker/docker/pull/8046/files#r19579960)

Signed-off-by: Vincent Batts <vbatts@redhat.com>
@LK4D4 no particular point, except that syscall.Stat_t are starting to bug me on how pervasive they are and as we add additional platforms we'll need a clean standard way to do the type assertions on `os.FileInfo.Sys()`, and switches do that better. That was my train of thought, though for a single case, I guess it looks silly. :-\