blk-mq: skip unmapped queues in blk_mq_alloc_request_hctx

This provides the caller a feedback that a given hctx is not mapped and thus
no command can be sent on it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Jens Axboe <axboe@fb.com>