RDMA/srpt: Report the SCSI residual to the initiator

The code added by this patch is similar to the code that already exists in
ibmvscsis_determine_resid(). This patch has been tested by running the
following command:

strace sg_raw -r 1k /dev/sdb 12 00 00 00 60 00 -o inquiry.bin |&
    grep resid=

Link: https://lore.kernel.org/r/20191105214632.183302-1-bvanassche@acm.org


Fixes: a42d985b ("ib_srpt: Initial SRP Target merge for v3.3-rc1")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Acked-by: Honggang Li <honli@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>