scsi: megaraid_sas: raid 1 write performance for large io

scsi: megaraid_sas: raid 1 write performance for large io

Avoid Host side PCI bandwidth bottleneck and hint FW to do Write
buffering using RaidFlag MR_RAID_FLAGS_IO_SUB_TYPE_LDIO_BW_LIMIT.  Once
IO is landed in FW with MR_RAID_FLAGS_IO_SUB_TYPE_LDIO_BW_LIMIT, it will
do single DMA from host and buffer the Write operation. On back end, FW
will DMA same buffer to the Mirror and Data Arm.  This will improve
large block IO performance which bottleneck due to Host side PCI
bandwidth limitation.

Consistent ~4000MB T.P for 256K Block size is expected performance
numbers.  IOPS for small Block size should be on par with Disk
performance.  (E.g 42 SAS Disk in JBOD mode gives 3700MB T.P.  Same
Drive used in R1 WT mode, should give ~1800MB T.P)

Using this patch 24 R1 VDs (HDD) gives below performance for Sequential
Write.  Without this patch, we cannot reach above 3200MB (Throughput is
in MB.)

Block Size   	50% 256K and 50% 4K          100% 256K
4K                 3100                        2030
8K                 3140                        2740
16K                3140                        3140
32K                3400                        3240
64K                3500                        3700
128K               3870                        3870
256K               3920                        3920

Signed-off-by: Shivasharan S <shivasharan.srikanteshwara@broadcom.com>
Signed-off-by: Kashyap Desai <kashyap.desai@broadcom.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Tomas Henzl <thenzl@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>