net: add a sysctl to reflect the fwmark on replies

Kernel-originated IP packets that have no user socket associated
with them (e.g., ICMP errors and echo replies, TCP RSTs, etc.)
are emitted with a mark of zero. Add a sysctl to make them have
the same mark as the packet they are replying to.

This allows an administrator that wishes to do so to use
mark-based routing, firewalling, etc. for these replies by
marking the original packets inbound.

Tested using user-mode linux:
 - ICMP/ICMPv6 echo replies and errors.
 - TCP RST packets (IPv4 and IPv6).

Conflicts:
	Documentation/networking/ip-sysctl.txt
	net/ipv6/tcp_ipv6.c

Change-Id: I1a3553ebea26dbc988272671cfa49f9a04dac520
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>