selftests/net: replace manual array size calc with ARRAYSIZE macro.

fixes coccinelle WARNING: Use ARRAY_SIZE

Signed-off-by: Mahmoud Maatuq <mahmoudmatook.mm@gmail.com>
Link: https://lore.kernel.org/r/20230716184349.2124858-1-mahmoudmatook.mm@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>