bonding: count master 3ad stats separately

I made a dumb mistake when I summed up the slave stats, obviously slaves
can come and go which would make the master stats unreliable.
Count and export the master stats separately.

Fixes: a258aeac

 ("bonding: add support for xstats and export 3ad stats")
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>