xprtrdma: Return an errno from rpcrdma_register_external()

The RPC/RDMA send_request method and the chunk registration code
expects an errno from the registration function. This allows
the upper layers to distinguish between a recoverable failure
(for example, temporary memory exhaustion) and a hard failure
(for example, a bug in the registration logic).
Signed-off-by: NChuck Lever <chuck.lever@oracle.com>
Signed-off-by: NAnna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/verbs.c

@@ -1918,10 +1918,10 @@ rpcrdma_register_external(struct rpcrdma_mr_seg *seg,
 		break;
 
 	default:
-		return -1;
+		return -EIO;
 	}
 	if (rc)
-		return -1;
+		return rc;
 
 	return nsegs;
 }