netns: fix ip_rt_frag_needed rt_is_expired

Running recent kernels, and using a particular vpn gateway, I've been
having to edit my mails down to get them accepted by the smtp server.

Git bisect led to commit e84f84f2

 -
netns: place rt_genid into struct net.  The conversion from a != test
to rt_is_expired() put one negative too many: and now my mail works.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Acked-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>