iprule: Add DSCP support

iprule: Add DSCP support

Add support for 'dscp' selector in ip-rule.

Rules can be added with a numeric DSCP value:

 # ip rule add dscp 1 table 100
 # ip rule add dscp 0x02 table 200

Or using symbolic names from /usr/share/iproute2/rt_dsfield or
/etc/iproute2/rt_dsfield:

 # ip rule add dscp AF42 table 300

Dump output:

 $ ip rule show
 0:      from all lookup local
 32763:  from all lookup 300 dscp AF42
 32764:  from all lookup 200 dscp 2
 32765:  from all lookup 100 dscp 1
 32766:  from all lookup main
 32767:  from all lookup default

Dump can be filtered by DSCP value:

 $ ip rule show dscp 1
 32765:  from all lookup 100 dscp 1

Or by a symbolic name:

 $ ip rule show dscp AF42
 32763:  from all lookup 300 dscp AF42

When the numeric option is specified, symbolic names will be translated
to numeric values:

 $ ip -N rule show
 0:      from all lookup 255
 32763:  from all lookup 300 dscp 36
 32764:  from all lookup 200 dscp 2
 32765:  from all lookup 100 dscp 1
 32766:  from all lookup 254
 32767:  from all lookup 253

The same applies to the JSON output in order to be consistent with
existing fields such as "tos" and "table":

 $ ip -j -p rule show dscp AF42
 [ {
         "priority": 32763,
         "src": "all",
         "table": "300",
         "dscp": "AF42"
     } ]

 $ ip -j -p -N rule show dscp AF42
 [ {
         "priority": 32763,
         "src": "all",
         "table": "300",
         "dscp": "36"
     } ]

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Daniel Machon <daniel.machon@microchip.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David Ahern <dsahern@kernel.org>