[NETNS][FRAGS]: Move ctl tables around.

This is a preparation for sysctl netns-ization.
Move the ctl tables to the files, where the tuning
variables reside. Plus make the helpers to register
the tables.

This will simplify the later patches and will keep
similar things closer to each other.

ipv4, ipv6 and conntrack_reasm are patched differently,
but the result is all the tables are in appropriate files.

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