rxrpc: Fix rxrpc_bundle::alloc_error to be signed

The alloc_error field in the rxrpc_bundle struct should be signed as it has
negative error codes assigned to it.  Checks directly on it may then fail,
and may produce a warning like this:

	net/rxrpc/conn_client.c:662 rxrpc_wait_for_channel()
	warn: 'bundle->alloc_error' is unsigned

Fixes: 245500d8 ("rxrpc: Rewrite the client connection manager")
Reported-by Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David Howells <dhowells@redhat.com>