./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>
Merge pull request #8863 from vbatts/vbatts-archive_stat

./pkg/archive: clean up Stat_t assertion