net: openvswitch: Use struct_size()

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

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Eelco Chaudron <echaudro@redhat.com>
Link: https://lore.kernel.org/r/e7746fbbd62371d286081d5266e88bbe8d3fe9f0.1683388991.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Jakub Kicinski <kuba@kernel.org>