tests: fail when "make mod-check" detects a modified go.mod file
tests: fail when "make mod-check" detects a modified go.mod file

`go mod verify` does not fail when all moduled under vendor/ are
updated. However it does update `go.mod` in case it does not reflect the
downloaded versions.

By adding this additional verification in `make mod-check`, future
updates to vendor/ will require a correct go.mod as well.

Signed-off-by: Niels de Vos <7bda0c0ee7f47171cc808e9e60292cbc5d8140dd@redhat.com>

ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi,ceph/ceph-csi