net: openvswitch: Use struct_size()

Use struct_size() instead of hand writing it.
This is less verbose and more robust.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/e5122b4ff878cbf3ed72653a395ad5c4da04dc1e.1697264974.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Paolo Abeni <pabeni@redhat.com>