nvme: introduce a nvme_host_path_error helper

When using nvme native multipathing, if a path related error occurs
during ->queue_rq, the request needs to be completed with
NVME_SC_HOST_PATH_ERROR so that the request can be failed over.

Introduce a helper to complete the command from ->queue_rq in a wait
that invokes nvme_complete_rq.

Signed-off-by: Chao Leng <lengchao@huawei.com>
[hch: renamed, added a return value to clean up the callers a bit]
Signed-off-by: Christoph Hellwig <hch@lst.de>