scsi: cxlflash: Support starting user contexts

User contexts request interrupts and are started using the "start work"
interface. Populate the start_work() fop to allocate and map interrupts before
starting the user context. As part of starting the context, update the user
process identification logic to properly derive the data required by the
SPA. Also, introduce a skeleton interrupt handler using a bitmap, flag, and
spinlock to track interrupts. This handler will be expanded in future commits.

Signed-off-by: Uma Krishnan <ukrishn@linux.vnet.ibm.com>
Acked-by: Matthew R. Ochs <mrochs@linux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>