RDMA/rxe: Close a race after ib_register_device

Since rxe allows unregistration from other threads the rxe pointer can
become invalid any moment after ib_register_driver returns. This could
cause a user triggered use after free.

Add another driver callback to be called right after the device becomes
registered to complete any device setup required post-registration.  This
callback has enough core locking to prevent the device from becoming
unregistered.

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

include/rdma/ib_verbs.h

@@ -2539,6 +2539,11 @@ struct ib_device_ops {
 			      struct rdma_restrack_entry *entry);
 
 	/* Device lifecycle callbacks */
+	/*
+	 * Called after the device becomes registered, before clients are
+	 * attached
+	 */
+	int (*enable_driver)(struct ib_device *dev);
 	/*
 	 * This is called as part of ib_dealloc_device().
 	 */