nvme-multipath: factor out a nvme_path_is_disabled helper

Factor our a common helper to check if a path has been disabled
by something other than the per-namespace ANA state.

Signed-off-by: Hannes Reinecke <hare@suse.com>
[hch: split from a bigger patch]
Signed-off-by: Christoph Hellwig <hch@lst.de>