[IPV6]: Don't update ADVMSS on routes where the MTU is not also updated

The ADVMSS value was incorrectly updated for ALL routes when the MTU
is updated because it's outside the effect of the if statement's
condition.

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Signed-off-by: David S. Miller <davem@davemloft.net>