scsi: cxlflash: Add kref to context

Currently, context user references are tracked via the list of LUNs that
have attached to the context. While convenient, this is not intuitive
without a deep study of the code and is inconsistent with the existing
reference tracking patterns within the kernel. This design choice can
lead to future bug injection.

To improve code comprehension and better protect against future bugs,
add explicit reference counting to contexts and migrate the context
removal code to the kref release handler.

Inspired-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Matthew R. Ochs <mrochs@linux.vnet.ibm.com>
Acked-by: Manoj N. Kumar <manoj@linux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>