nvme-multipath: sysfs links may not be created for devices

When rapidly rescanning for new namespaces nvme_mpath_add_sysfs_link() may be
called for a block device not added to sysfs. But NVME_NS_SYSFS_ATTR_LINK
had already been set, so when checking this device a second time we will fail
to create the link.

Fix this by exchanging the order of the block device check and the
NVME_NS_SYSFS_ATTR_LINK bit check.

Fixes: 4dbd2b2e ("nvme-multipath: Add visibility for round-robin io-policy")
Signed-off-by: Hannes Reinecke <hare@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me&gt;**>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>