AF_PACKET: Add documentation for queue mapping fanout mode

Recently I added a new AF_PACKET fanout operation mode in commit
2d36097d, but I forgot to document it.  Add PACKET_FANOUT_QM as an available mode
in the af_packet documentation.  Applies to net-next.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
CC: "David S. Miller" <davem@davemloft.net>
CC: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>