ip: bridge: add xstats json support

ip: bridge: add xstats json support

Add json support for bridge's xstats output.
The plain text output format should remain the same.
Note that this patch pulls the interface out of the attribute
loop, this was an oversight when the set was upstreamed. This does not
change the output format, but fixes it when new xstats attributes show
up.

Example:
$ ip -p -j link xstats type bridge
  [ {
        "ifname": "br0",
        "multicast": {
            "igmp_queries": {
                "rx_v1": 0,
                "rx_v2": 32,
                "rx_v3": 0,
                "tx_v1": 0,
                "tx_v2": 0,
                "tx_v3": 0
            },
            "igmp_reports": {
                "rx_v1": 0,
                "rx_v2": 32,
                "rx_v3": 0,
                "tx_v1": 0,
                "tx_v2": 0,
                "tx_v3": 0
            },
            "igmp_leaves": {
                "rx": 0,
                "tx": 0
            },
            "igmp_parse_errors": 0,
            "mld_queries": {
                "rx_v1": 33,
                "rx_v2": 0,
                "tx_v1": 0,
                "tx_v2": 0
            },
            "mld_reports": {
                "rx_v1": 66,
                "rx_v2": 2,
                "tx_v1": 0,
                "tx_v2": 0
            },
            "mld_leaves": {
                "rx": 0,
                "tx": 0
            },
            "mld_parse_errors": 0
        }
    } ]

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David Ahern <dsahern@gmail.com>