dm init: fix incorrect uses of kstrndup()

Fix 2 kstrndup() calls with incorrect argument order.

Fixes: 6bbc923d ("dm: add support to directly boot to a mapped device")
Cc: stable@vger.kernel.org # v5.1
Signed-off-by: Gen Zhang <blackgod016574@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>