nvme-multipath: drop head pointer check in nvme_mpath_clear_current_path()

nvme-multipath: drop head pointer check in nvme_mpath_clear_current_path()

A NS will always have a head pointer, so drop the check. As proof in
practice, all the nvme_mpath_clear_current_path() callers also
dereference ns->head.

This check has endured since the original changes to support multipath.

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>