ipv4: Remove output route check in ipv4_mtu

The output route check was introduced with git commit 261663b0


(ipv4: Don't use the cached pmtu informations for input routes)
during times when we cached the pmtu informations on the
inetpeer. Now the pmtu informations are back in the routes,
so this check is obsolete. It also had some unwanted side effects,
as reported by Timo Teras and Lukas Tribus.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Acked-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>