flow: Refactor flow_compose() API.

flow: Refactor flow_compose() API.

Currently, flow_compose_size() is only supposed to be called after
flow_compose(). I find this API to be unintuitive.

Change flow_compose() API to take the 'size' argument, and
returns 'true' if the packet can be created, 'false' otherwise.

This change also improves error detection and reporting when
'size' is unreasonably small.

Signed-off-by: Andy Zhou <azhou@ovn.org>
Acked-by: Ilya Maximets <i.maximets@samsung.com>