IB/srp: Fix srp_map_sg_dma()

Because patch "IB/srp: Move common code into the caller" was applied
partially srp_map_sg_dma() doesn't work properly. Fix this by
applying the remainder of that patch. See also
http://thread.gmane.org/gmane.linux.drivers.rdma/35803/focus=35811

.

Fixes: 3849e44d ("IB/srp: Move common code into the caller")
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Mike Marciniszyn <mike.marciniszyn@intel.com>
Cc: Sagi Grimberg <sai@grimberg.me>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Laurence Oberman <loberman@redhat.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Doug Ledford <dledford@redhat.com>