IB/core: Remove overload in ib_sg_dma*

The code is replaced by driver specific changes and avoids the pointer
NULL test for drivers that don't overload these operations.

Suggested-by: &lt;Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Tested-by: Vinod Kumar <vinod.kumar@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>