RDMA: Constify the argument of the work request conversion functions

When posting a send work request, the work request that is posted is not
modified by any of the RDMA drivers. Make this explicit by constifying
most ib_send_wr pointers in RDMA transport drivers.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>