IB/core: Introduce and use rdma_gid_table()

There are several places a gid table is accessed.
Have a helper tiny function rdma_gid_table() to avoid code
duplication at such places.

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