RDMA/core: Add helper function to retrieve driver gid context from gid attr

Adding a helper function to retrieve the driver gid context from the gid
attr.

Link: https://lore.kernel.org/r/1582107594-5180-2-git-send-email-selvin.xavier@broadcom.com


Suggested-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Selvin Xavier <selvin.xavier@broadcom.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>