ipv4: Do not cache routing failures due to disabled forwarding.

If we cache them, the kernel will reuse them, independently of
whether forwarding is enabled or not.  Which means that if forwarding is
disabled on the input interface where the first routing request comes
from, then that unreachable result will be cached and reused for
other interfaces, even if forwarding is enabled on them.  The opposite
is also true.

This can be verified with two interfaces A and B and an output interface
C, where B has forwarding enabled, but not A and trying
ip route get $dst iif A from $src && ip route get $dst iif B from $src

Signed-off-by: Nicolas Cavallari <nicolas.cavallari@green-communications.fr>
Reviewed-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: David S. Miller <davem@davemloft.net>