[NET]: Make the netlink methods in rtnetlink handle multiple network namespaces

[NET]: Make the netlink methods in rtnetlink handle multiple network namespaces

After the previous prep work this just consists of removing checks
limiting the code to work in the initial network namespace, and
updating rtmsg_ifinfo so we can generate events for devices in
something other then the initial network namespace.

Referring to network other network devices like the IFLA_LINK
and IFLA_MASTER attributes do, gets interesting if those network
devices happen to be in other network namespaces.  Currently
ifindex numbers are allocated globally so I have taken the path
of least resistance and not still report the information even
though the devices they are talking about are invisible.

If applications start getting confused or when ifindex
numbers become local to the network namespace we may need
to do something different in the future.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Denis V. Lunev <den@openz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>