net: ipv4: route.c: simplify procfs code

proc_creat_seq() that directly take a struct seq_operations,
and deal with network namespaces in ->open.

Signed-off-by: Yejune Deng <yejune.deng@gmail.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>