pkg/mount: include optional field

one linux, the optional field designates the sharedsubtree information,
if any.

Signed-off-by: Vincent Batts <vbatts@redhat.com>
pkg/mount: testing mountinfo fields

Signed-off-by: Vincent Batts <vbatts@redhat.com>
Merge pull request #8937 from vbatts/vbatts-mount_optional_fields

pkg/mount: include optional field