IB/core: Add a sgid_attr pointer to struct rdma_ah_attr

The sgid_attr will ultimately replace the sgid_index in the ah_attr.
This will allow for all layers to have a consistent view of what
gid table entry was selected as processing runs through all stages of the
stack.

This commit introduces the pointer and ensures it is set before calling
any driver callback that includes a struct ah_attr callback, allowing
future patches to adjust both the drivers and the callers to use
sgid_attr instead of sgid_index.

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