iprule: Add DSCP mask support

iprule: Add DSCP mask support

Add DSCP mask support, allowing users to specify a DSCP value with an
optional mask. Example:

 # ip rule add dscp 1 table 100
 # ip rule add dscp 0x02/0x3f table 200
 # ip rule add dscp AF42/0x3f table 300
 # ip rule add dscp 0x10/0x30 table 400

In non-JSON output, the DSCP mask is not printed in case of exact match
and the DSCP value is printed in hexadecimal format in case of inexact
match:

 $ ip rule show
 0:      from all lookup local
 32762:  from all lookup 400 dscp 0x10/0x30
 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 and mask:

 $ ip rule show dscp 1
 32765:  from all lookup 100 dscp 1
 $ ip rule show dscp AF42
 32763:  from all lookup 300 dscp AF42
 $ ip rule show dscp 0x10/0x30
 32762:  from all lookup 400 dscp 0x10/0x30

In JSON output, the DSCP mask is printed as an hexadecimal string to be
consistent with other masks. The DSCP value is printed as an integer in
order not to break existing scripts:

 $ ip -j -p -N rule show dscp 0x10/0x30
 [ {
         "priority": 32762,
         "src": "all",
         "table": "400",
         "dscp": "16",
         "dscp_mask": "0x30"
     } ]

The mask attribute is only sent to the kernel in case of inexact match
so that iproute2 will continue working with kernels that do not support
the attribute.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David Ahern <dsahern@kernel.org>