IB: Ensure that all rdma_ah_attr's are zero initialized

Since we are adding some new fields to this structure it is safest if all
users reliably initialize the struct to zero.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>