RDMA/cma: Introduce API to read GIDs for multiple transports

This patch introduces an API that allows legacy applications to query
GIDs for a rdma_cm_id which is used during connection establishment.

GIDs are stored and created differently for iWarp, IB and RoCE transports.
Therefore rdma_read_gids() returns GID for all the transports hiding
such internal details to caller.
It is usable for client side and server side connections.

In general continued use of GID based addressing outside of IB is
discouraged, so rdma_read_gids() should not be used by any new ULPs.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>