oci: resolve cross-filesystem blob caching failure

oci: resolve cross-filesystem blob caching failure

This commit addresses an issue in the OCI template where lxc-create
fails if OCI-cache directory for blob caching is not on the same mount
as the destination OCI directory. lxc-create bails when skopeo tries to
create a hard-link across the two and fails.

For example, if /var/lib/lxc is a bind mountpoint of a random directory
and skopeo fails to hard-link across /var/cache/lxc and /var/lib/lxc

This commit introduces a check where if both directories are on not the
same mount points, it disables blob caching in skopeo and continues.

Signed-off-by: Tanmay Naik <tnaik96@gmail.com>