Create: Fix checking for container in update_metadata

Create: Fix checking for container in update_metadata

The commit 8a4ce2c05386 ("Create: Factor out add_disks() helpers")
introduced a regression that caused timeouts and udev failing to create
links.

Steps to reproduce the issue were as following:
$ mdadm -CR imsm -e imsm -n4 /dev/nvme[0-3]n1
$ mdadm -CR vol -l5 -n4 /dev/nvme[0-3]n1 --assume-clean

I found the check for container was wrong because negation was missing.

Fixes: 8a4ce2c05386 ("Create: Factor out add_disks() helpers")
Signed-off-by: Mateusz Grzonka <mateusz.grzonka@intel.com>
Signed-off-by: Jes Sorensen <jes@trained-monkey.org>