IB/core: Fix no default GIDs when netdevice reregisters

Currently, when the netdevice returned by get_netdev is unregistered,
we delete all GIDs (including the default GIDs) and reset their
attributes. Therefore, when we re-register it, no default GIDs
will be assigned (as their "default GID") attribute will be reset.
Fixing this by keeping "default GID" attribute.

Fixes: 03db3a2d ('IB/core: Add RoCE GID table management')
Signed-off-by: Talat Batheesh <talatb@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>