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).

Change-Id: I6873d973196797bcf32e2e91976df647c7e8b85a
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Git-commit: 5a87fa6a43733e241406e8d62fe28fdc0735bf93
Git-repo: https://android.googlesource.com/kernel/common.git


[imaund@codeaurora.org: Resolve trivial merge conflicts]
Signed-off-by: Ian Maund <imaund@codeaurora.org>