RDMA: Hold the sgid_attr inside the struct ib_ah/qp

If the AH has a GRH then hold a reference to the sgid_attr inside the
common struct.

If the QP is modified with an AV that includes a GRH then also hold a
reference to the sgid_attr inside the common struct.

This informs the cache that the sgid_index is in-use so long as the AH or
QP using it exists.

This also means that all drivers can access the sgid_attr directly from
the ah_attr instead of querying the cache during their UD post-send paths.

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