net: move procfs code to net/core/net-procfs.c

Similar to net/core/net-sysfs.c, group procfs code to
a single unit.

Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>