iplink: add support for reporting multiple XDP programs

iplink: add support for reporting multiple XDP programs

Kernel now supports attaching XDP programs in the driver
and hardware at the same time.  Print that information
correctly.

In case there are multiple programs attached kernel will
not provide IFLA_XDP_PROG_ID, so don't expect it to be
there (this also improves the printing for very old kernels
slightly, as it avoids unnecessary "prog/xdp" line).

In short mode preserve the current outputs but don't print
IDs if there are multiple.

6: netdevsim0: <BROADCAST,NOARP> mtu 1500 xdpoffload/id:11 qdisc [...]

and:

6: netdevsim0: <BROADCAST,NOARP> mtu 1500 xdpmulti qdisc [...]

ip link output will keep using prog/xdp prefix if only one program
is attached, but can also print multiple program lines:

    prog/xdp id 8 tag fc7a51d1a693a99e jited

vs:

    prog/xdpdrv id 8 tag fc7a51d1a693a99e jited
    prog/xdpoffload id 9 tag fc7a51d1a693a99e

JSON output gains a new array called "attached" which will
contain the full list of attached programs along with their
attachment modes:

        "xdp": {
            "mode": 3,
            "prog": {
                "id": 11,
                "tag": "fc7a51d1a693a99e",
                "jited": 0
            },
            "attached": [ {
                    "mode": 3,
                    "prog": {
                        "id": 11,
                        "tag": "fc7a51d1a693a99e",
                        "jited": 0
                    }
                } ]
        },

In case there are multiple programs attached the general "xdp"
section will not contain program information:

        "xdp": {
            "mode": 4,
            "attached": [ {
                    "mode": 1,
                    "prog": {
                        "id": 10,
                        "tag": "fc7a51d1a693a99e",
                        "jited": 1
                    }
                },{
                    "mode": 3,
                    "prog": {
                        "id": 11,
                        "tag": "fc7a51d1a693a99e",
                        "jited": 0
                    }
                } ]
        },

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David Ahern <dsahern@gmail.com>