route-table: Stop netlink log message when routes withdrawn

route-table: Stop netlink log message when routes withdrawn

When a route is withdrawn (blackholed) the netlink message doesn't include
an RTA_OIF element. This results in an "unexpected netlink message
contents" log message because this element is not optional.

Given that the netlink message will be ignored anyway, and subsequent
error checking will cope with missing RTA_OIF, the element should be
optional in order to suppress unnecessary log messages.

Signed-off-by: Tony van der Peet <tony.vanderpeet@alliedtelesis.co.nz>
Signed-off-by: Ben Pfaff <blp@ovn.org>