RDMA: Convert drivers to use the AH's sgid_attr in post_wr paths

For UD the drivers were doing a sgid_index lookup into the cache to get
the attrs, however we can now directly access the same attrs stores in
the ib_ah instead and remove the lookup.

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