We maintain global statistics for an entire MDIO bus, as well as broken
down, per MDIO bus address statistics. Given that it is possible for
MDIO devices such as switches to access MDIO bus addresses for which
there is not a mdio_device instance created (therefore not a a
corresponding device directory in sysfs either), we also maintain
per-address statistics under the statistics folder. The layout looks
like this:

/sys/class/mdio_bus/../statistics/
	transfers
	errrors
	writes
	reads
	transfers_<addr>
	errors_<addr>
	writes_<addr>
	reads_<addr>

When a mdio_device instance is registered, a statistics/ folder is
created with the tranfers, errors, writes and reads attributes which
point to the appropriate MDIO bus statistics structure.

Statistics are 64-bit unsigned quantities and maintained through the
u64_stats_sync.h helper functions.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>