IB: Make init_ah_attr_grh_fields set sgid_attr

Use the sgid and other information from the path record to figure out the
sgid_attrs.

Store the selected table entry in the sgid_attr for everything else to
use.

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