scsi: core: Inline scsi_mq_alloc_queue()

Since scsi_mq_alloc_queue() only has one caller, inline it. This change was
suggested by Christoph Hellwig.

Link: https://lore.kernel.org/r/20210622024654.12543-1-bvanassche@acm.org
Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Ed Tsai <ed.tsai@mediatek.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>