IB/core: Rename ib_modify_ah to rdma_modify_ah

Rename ib_modify_ah to rdma_modify_ah so its in sync with the
rename of the ib address handle attribute

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Don Hiatt <don.hiatt@intel.com>
Reviewed-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Dasaratharaman Chandramouli <dasaratharaman.chandramouli@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>