tc: flower: document that *_ip parameters take a PREFIX as an argument.

tc: flower: document that *_ip parameters take a PREFIX as an argument.

* The argument to src_ip, dst_ip, enc_src_ip and enc_dst_ip take an
  optional prefix length which is used to provide a mask to limit the scope
  of matching.
* This is documented as a PREFIX in keeping with ip-route(8).

Example of uses of IPv4 and IPv6 prefixes

tc qdisc add dev eth0 ingress
tc filter add dev eth0 protocol ip parent ffff: flower \
    indev eth0 dst_ip 192.168.1.1 action drop
tc filter add dev eth0 protocol ip parent ffff: flower \
    indev eth0 src_ip 10.0.0.0/8 action drop
tc filter add dev eth0 protocol ipv6 parent ffff: flower \
    indev eth0 src_ip 2001:DB8:1::/48 action drop
tc filter add dev eth0 protocol ipv6 parent ffff: flower \
    indev eth0 dst_ip 2001:DB8::1 action drop

Signed-off-by: Simon Horman <simon.horman@netronome.com>