scsi: megaraid_sas: reduce size of fusion_context and use kmalloc for allocation

fusion_context structure is very large around 180kB and most of the size
is contributed by log_to_span array. Move log_to_span out of fusion
context and have separate allocation for log_to_span. And use kmalloc to
allocate fusion_context.  Currently kmemleak reports 1000s of false
positives for fusion->cmd_list[]. kmemleak does not track page
allocation for fusion_context. This change will also fix the false
positives reported by kmemleak.

Ref: https://marc.info/?l=linux-scsi&m=150545293900917



Reported-by: Shu Wang <shuwang@redhat.com>
Signed-off-by: Kashyap Desai <kashyap.desai@broadcom.com>
Signed-off-by: Shivasharan S <shivasharan.srikanteshwara@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>