[AFS]: Add support for the CB.GetCapabilities operation.

[AFS]: Add support for the CB.GetCapabilities operation.

Add support for the CB.GetCapabilities operation with which the fileserver can
ask the client for the following information:

 (1) The list of network interfaces it has available as IPv4 address + netmask
     plus the MTUs.

 (2) The client's UUID.

 (3) The extended capabilities of the client, for which the only current one
     is unified error mapping (abort code interpretation).

To support this, the patch adds the following routines to AFS:

 (1) A function to iterate through all the network interfaces using RTNETLINK
     to extract IPv4 addresses and MTUs.

 (2) A function to iterate through all the network interfaces using RTNETLINK
     to pull out the MAC address of the lowest index interface to use in UUID
     construction.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>