IB/core: Introduce GID attribute get, put and hold APIs

This patch introduces three APIs, rdma_get_gid_attr(),
rdma_put_gid_attr(), and rdma_hold_gid_attr() which expose the reference
counting for GID table entries to the entire stack. The kref counting is
based on the struct ib_gid_attr pointer

Later patches will convert more cache query function to return struct
ib_gid_attrs.

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