Using securejoin.SecureJoin() ensures that paths are bound within a
given root, but it doesn't protect from changes happening between the
construction of the path and its use. securejoin 0.3 introduces a new
Linux-specific API which avoids this by making rooted open operations
explicit; this migrates kubelet's log retrieval to use that.
Signed-off-by: Stephen Kitt <skitt@redhat.com>
