nvme: avoid creating multipath sysfs group under namespace path devices

Commit 4dbd2b2e ("nvme-multipath: Add visibility for round-robin
io-policy") introduced the creation of the multipath sysfs group under
the NVMe head gendisk device node. However, it also inadvertently added
the same sysfs group under each namespace path device which head node
refers to and that is incorrect.

The multipath sysfs group should only be exposed through the namespace
head gendisk node. This is sufficient, as the head device already
provides symbolic links to the individual namespace paths it manages.

This patch fixes the issue by preventing the creation of the multipath
sysfs group under namespace path devices, ensuring it only appears under
the head disk node.

Fixes: 4dbd2b2e ("nvme-multipath: Add visibility for round-robin io-policy")
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>