RDMA/ucma: Return stable IB device index as identifier

The librdmacm uses node_guid as identifier to correlate between IB devices
and CMA devices. However FW resets cause to such "connection" to be lost
and require from the user to restart its application.

Extend UCMA to return IB device index, which is stable identifier.

Link: https://lore.kernel.org/r/20200504132541.355710-1-leon@kernel.org


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