RDMA/core: Introduce rdma_read_gid_attr_ndev_rcu() to check GID attribute

Introduce an API rdma_read_gid_attr_ndev_rcu() to return GID attribute
netdevice which is in UP state for accessing netdevice's fields such as
net namespace and ifindex.

This is useful for users who intent to access netdevice fields under rcu
lock.

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