net: bridge: add STP xstats

This adds rx_bpdu, tx_bpdu, rx_tcn, tx_tcn, transition_blk,
transition_fwd xstats counters to the bridge ports copied over via
netlink, providing useful information for STP.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>