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>

Documentation/networking/packet_mmap.txt

@@ -583,6 +583,7 @@ Currently implemented fanout policies are:
   - PACKET_FANOUT_CPU: schedule to socket by CPU packet arrives on
   - PACKET_FANOUT_RND: schedule to socket by random selection
   - PACKET_FANOUT_ROLLOVER: if one socket is full, rollover to another
+  - PACKET_FANOUT_QM: schedule to socket by skbs recorded queue_mapping
 
 Minimal example code by David S. Miller (try things like "./test eth0 hash",
 "./test eth0 lb", etc.):