IB/rxe: Initialize ib_ah_attr during query_ah

Zero out ib_ah_attr before calling query_ah. Set ah_flags
appropriately.

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>