ipv6: fib: Restore NTF_ROUTER exception in fib6_age()

Commit 5339ab8b

 (ipv6: fib: Convert fib6_age() to
dst_neigh_lookup().) seems to have mistakenly inverted the
exception for cached NTF_ROUTER routes.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>