RDMA/srp: Handle large SCSI CDBs correctly

Reserve additional space for CDBs that contain more than sixteen bytes
and set the add_cdb_len field for such CDBs as required. From the SRP
standard: "The ADDITIONAL CDB LENGTH field contains the length in
dwords of the ADDITIONAL CDB field."

Cc: Sergey Gorenko <sergeygo@mellanox.com>
Cc: Max Gurtovoy <maxg@mellanox.com>
Cc: Laurence Oberman <loberman@redhat.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>