RDMA/siw: Fabricate a GID on tun and loopback devices

LOOPBACK and NONE (tunnel) devices have all-zero MAC addresses.
Currently, siw_device_create() falls back to copying the IB device's
name in those cases, because an all-zero MAC address breaks the RDMA
core address resolution mechanism.

However, at the point when siw_device_create() constructs a GID, the
ib_device::name field is uninitialized, leaving the MAC address to
remain in an all-zero state.

Fabricate a random artificial GID for such devices, and ensure this
artificial GID is returned for all device query operations.

Link: https://lore.kernel.org/r/168960673260.3007.12378736853793339110.stgit@manet.1015granger.net
Reported-by: Tom Talpey <tom@talpey.com>
Fixes: a2d36b02

 ("RDMA/siw: Enable siw on tunnel devices")
Reviewed-by: Bernard Metzler <bmt@zurich.ibm.com>
Reviewed-by: Tom Talpey <tom@talpey.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>