nvme-multipath: pass NS head to nvme_mpath_revalidate_paths()

nvme-multipath: pass NS head to nvme_mpath_revalidate_paths()

In nvme_mpath_revalidate_paths(), we are passed a NS pointer and use that
to lookup the NS head and then use that same NS pointer as an iter variable.

It makes more sense pass the NS head and use a local variable for the NS
iter.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>