net: Drop @ri test but use known ARRA_SIZE

We might miss entry in "ri ? ri - 1" expression when ri = 1.
Lets use known array size instead.

For some reason it didn't trigger on my tests earlier.

Reported-by: Andrew Vagin <avagin@odin.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Andrew Vagin <avagin@odin.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>