RDMA/rxe: Use driver_unregister and new unregistration API

rxe does not have correct locking for its registration/unregistration
paths, use the core code to handle it instead. In this mode
ib_unregister_device will also do the dealloc, so rxe is required to do
clean up from a callback.

The core code ensures that unregistration is done only once, and generally
takes care of locking and concurrency problems for rxe.

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