inet: Rename fragmentation sysctl-related functions/variables.

The fragments sysctls also contains some, that are to be
visible, but read-only in net namespaces.

The naming in net/core/sysctl_net_core.c is - tables, that are
to be registered in namespaces have a "ns" word in their names.
So rename ones in ipv4/ip_fragment.c and ipv6/reassembly.c to
fit this.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>