mpls: Basic routing support

This change adds a new Kconfig option MPLS_ROUTING.

The core of this change is the code to look at an mpls packet received
from another machine.  Look that packet up in a routing table and
forward the packet on.

Support of MPLS over ATM is not considered or attempted here.  This
implemntation follows RFC3032 and implements the MPLS shim header that
can pass over essentially any network.

What RFC3021 refers to as the as the Incoming Label Map (ILM) I call
net->mpls.platform_label[].  What RFC3031 refers to as the Next Label
Hop Forwarding Entry (NHLFE) I call mpls_route.  Though calling it the
label fordwarding information base (lfib) might also be valid.

Further the implemntation forwards packets as described in RFC3032.
There is no need and given the original motivation for MPLS a strong
discincentive to have a flexible label forwarding path.  In essence
the logic is the topmost label is read, looked up, removed, and
replaced by 0 or more new lables and the sent out the specified
interface to it's next hop.

Quite a few optional features are not implemented here.  Among them
are generation of ICMP errors when the TTL is exceeded or the packet
is larger than the next hop MTU (those conditions are detected and the
packets are dropped instead of generating an icmp error).  The traffic
class field is always set to 0.  The implementation focuses on IP over
MPLS and does not handle egress of other kinds of protocols.

Instead of implementing coordination with the neighbour table and
sorting out how to input next hops in a different address family (for
which there is value).  I was lazy and implemented a next hop mac
address instead.  The code is simpler and there are flavor of MPLS
such as MPLS-TP where neither an IPv4 nor an IPv6 next hop is
appropriate so a next hop by mac address would need to be implemented
at some point.

Two new definitions AF_MPLS and PF_MPLS are exposed to userspace.

Decoding the mpls header must be done by first byeswapping a 32bit bit
endian word into the local cpu endian and then bit shifting to extract
the pieces.  There is no C bit-field that can represent a wire format
mpls header on a little endian machine as the low bits of the 20bit
label wind up in the wrong half of third byte.  Therefore internally
everything is deal with in cpu native byte order except when writing
to and reading from a packet.

For management simplicity if a label is configured to forward out
an interface that is down the packet is dropped early.  Similarly
if an network interface is removed rt_dev is updated to NULL
(so no reference is preserved) and any packets for that label
are dropped.  Keeping the label entries in the kernel allows
the kernel label table to function as the definitive source
of which labels are allocated and which are not.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>